Honda-SOHC

Interesting that DS show the original pipe p/n 17681-377-000 as no longer available.
I suspect the original was Imperial based like a lot of early Japanese Car Parts probably 13/64" I/d  so no direct equivalent available.

Have you split the crank case yet?
My priority would be checking crankshaft, crank bearings, primary drive system, gearbox bearings, oil pump, cam chain tensioner & pistons. 
The cost of ensuring the engine unit is A1 would then impact on choices such as new paintwork & exhaust & casing cosmetics.

I view a project overspend as normal depreciation - hopefully a one off cost in year 2 for accounting purposes. Alternatively just forget the over run. 

I'm stating the obvious, like you I am conscious that fettling  costs can easily out run end value. Money spent on the heart of the bike is money well spent. Do it once do it right.

ATM I use Ethanol Free SUL so not an issue.

Is there a difference in wall thickness?

Final thoughts if it's definitely a front wheel area issue flags up the following ideas.
Is the wheel rim  & tyre completely  round - try some visual checks with the front end jacked up - rotate wheel check against best means you have of looking for ovality.

Fork oil change you already have in mind.

I've no experience of effects of wheel bearing wear aside from noise - the front brake will make wear difficult to check.

Probably stating the obvious so just trying to give you my thoughts.

Yes a flat washer - that's why I thought it might be a fibre one!

I've taken onboard both those comments Ken for when I reach assembly on my 500 - the point about using a bolt rather than a JIS screw is a good one - I have the chrome bolt but might change it to SS.

Your sizes don't really add up but try some options here for size.
If they are 1.5mm wall thickness rings with an inside diameter of 10mm then the outside diameter would be 13mm.
If they are 1.5mm diameter with an outside diameter of 16mm then the inside diameter would be 13mm.

The ring shown in your diagram as 16mm o/d with 10mm i/d would be a 3mm thickness ring - this size is listed if you enter the details

I think the 16mm * 1.5 is the thread pitch.

https://www.polymax.co.uk/o-rings/rubber-viton-oring/