Honda-SOHC

That is good news Bryan, hopefully after the "rebore" you'll get no more trouble 

The technical points:- as you say Dave,  the bolts are a real problem if corrosion gets into the threads.  The zinc plating is the part that protects them originally,  but as we all know when exposed to salty winter roads it will strip that off over time.  The zinc is sacrificial so that when you see them going white in early stages of their life you are observing that process in operation. Once gone though,  it starts on the components to give that siezed stage that can destroy the parts. Using something to avoid this is sensible as you do with coppaslip,  just have to be aware of the effect on measured torque.

Bryan picked up on scale with things like this.  it seems to trip up the unaware in not acknowledging the very small scale that is involved, many just don't believe that they are working with such small numbers, for both tolerancing and torque settings.

To elaborate on the cam cover bolts Simon, yes it should be true that if never over torqued they should be ok for most eventuality.  They have more to them than is generally appreciated I believe. I mentioned earlier about the torque on the threads,  what Honda have done on these though is to design in a wide head/flange on the bolt. The increase in the surface area is the principal element in how the torque is built up.  If tightened correctly it'll reach the measured specification just from this surface and not overstress the thread area. As mentioned,  the joint face is not compressed like a gasket,  so once the two metal components come into contact that is job done,  the bolts just need the torque setting to stop them backing out during use.
It's one of those assemblies in which some very subtle elements combine to give an integrated whole component.

Retightening head bolts,  it's one of those things that I don't think can be answered definitely as to method,  unless the original manufacture gives a prescribed routine.
Faced with trying to retourque them means it's already compromised the gasket to some extent,  else it would not be required. Agree with Bryan in releasing first then tightening again to get a correct reading.
I'd do them one at a time so that the head casting is not released and in the original pattern of engine build if doing them all.
It's one of those things that you'd hope to get it to seal,  but may have to accept it needs replacement if not effective.

That K-series engine Dave, as far as I remember they have bolts that go right through the whole assembly and hold crank main,  pass through block,  then the head to squeeze the whole assembly. It's quite a clever design,  but like many things considered in that way, sometimes mis-understood.
They are definitely "stretched" bolts, in that they are pulled during tightening past elasticity into primary yeald to set them correctly and need replacing if taken apart.
Great irony of those engines is it's not this construction that causes the fault in the first place,  but often gets blamed. The overheating usually originates with failure of water system sealing to the inlet manifold as I understand it. They run a fairly small coolant volume and will readily overheat because of this problem,  by which time the head gasket is compromised.

Reference the bolt/fixing integrity for rocker cover.

From an engineering point of view I don't feel that they wear out, as such.  It's the case that the steel bolts can very easily exceed any torque aluminium castings can tolerate.

In tightening them,  if the torque used succeeds in fracturing the threads ( in essence detaching the thread form from its root anchored to the surrounding metal)  then they are scrap from that point onwards. It doesn't matter if it's the first, or many times its been used.

That joint is unusual in how it's viewed from a technical point.  Using its two faces to provide the tolerancing for cam bearings,  it only has to be tightened until the surfaces touch by which time the o-ring shouod have long been compressed into its sealing function. The remainder of tightening after this point is just to provide a "shakeproof" torque on the bolts. It really doesn't need much to do this.

There's anther factor that's not really considered,  which is lubrication of the bolts and the affect it has on torque rating. I think Bryan made this observation about whether to lubricate the thread's some years ago (that was in relation to head studs, that I remember).
As I understand it,  thread torque used is given for a dry thread UNLESS a lubricant is specified.
If you add a lubricant to this equation it requires you to use LESS measured torque to achieve the same clamping force, but the load on each thread goes upwards toward failure. Torque is just measuring friction and has to be qualified for the friction in place.  Change the friction,  and you change the measurement of torque as it's rotary and specifically doesn't measure the clamping force.

It follows that if you use oil and maximum torque specified,  then you'll come closer to the materials capabilites to resist failure.

The general interpretation would be to reduce torque by 10 to 15% indicated by figures I can find. 

Very good rebuild thread Roy.

Impressed with how flat and perfectly sealed that original head gasket face was in the first picture above.

I feel the current (post asbestos)  organic pads have a very soft bite and sensitivity to them.  Even in cars they seem to need higher brake line pressure to get them generating significant force.

Honda seemed to use a very conservative leverage ratio on these systems, so quite hard to generate high line pressure. Jokingly I've considered these to be an early anti-lock brake system as you'd need hands like a bricky to put enough force into them.

The disc is easily big enough to make the torque required,  just that you can't exert sufficient pressure through the leverage ratio they use.

Contemporary systems to me seemed to have the Yamaha RD much closer to current systems, with much lower lever pressure able to generate quite potent braking.
The mastercylinder from later 400E, the one with floating calipers and single piston may offer an interesting combination with the Honda caliper on the 750 to test that theory.  It may give you an indication of what the disc and caliper could do.  Shame they are the square type otherwise would look less out of place.

Honda routinely use more or less bespoke bolts on many applications,  you really have to be quite careful if you are to replace any.

Friend had an XR500 that had one (or two) 9mm bolts in the head!

You sometimes get odd things from very competent engineering designs though.  Mercedes cars fitted with alloy wheels and steel spare used to need shorter wheel bolts to fit the spare.  If you used the "alloy" bolts they would trepan a neat groove in the brake caliper from extending too far through the hub.

And Peugeot have used five sided bolts on their brake calipers to prevent tampering in the past.

Just sometimes a specific accuracy gives rise to a unseen fault that the designers didn't spot.

Or this one

https://www.louis-moto.co.uk/artikel/wilbers-fork-oil-zero-friction/60927990?list=3b58a80b03a9f50490246b86f846ffe2

I haven't tried it,  and would guess it could be silicone based.  But they are obviously looking at this area of fork performance,  could give you help.

That's a bit of a result,  surprising what we keep in our garages.

There are some services offered as many archives used microfilm etc to store documents.

Search for "microfiche to digital " for services.  May be too commercial and involved for this scenario,  plus copyright potentially.

As a rough any ready test,  use masking tape to stick a sample to a window in daylight, then see what you can resolve with something like a phone camera at close range.

Ideally you need the smallest sized sensor (physical dimensions)  with the highest megapixel count, combined with the closest focus ability.

If you can get it onto the chip at something aproaching life size,  then the software will handle the viewing size. Plus being monochromatic,  it should give decent sharpening to the details.