Honda-SOHC

There I was hoping the culprit would be discovered and I could cure my gutless engine.
But those marks in the bores look very wrong as others note and tell me the compression will be down.
Hoping to find that its been the valve seats all along with mine.

So long as you can read it upside down

What a Carry On

With care it's not too tricky.
Good lighting helps too.
Get the throttle cables up and clear, out of the way of the pulley and cable bracket.
Flexible inlet rubbers (or new ones if possible) will help too, with the clips loosened off.
Also loosen the large clip between the manifold on the oval connector, then use a bungy to hold the manifold against the frame which gives a few mm extra clearance to ease the carb bank into position.
Angle the back of the carbs down as they are moved into position from the RHS, then insert them into the 4 insulator rubbers (again, clips loosened). Quite a tight fit so a tiny amount of WD applied by finger to each carb spigot helps.
The rubber connectors can then be adjusted/rotated correctly. I put a smidge of WD on the inside of each one to help them onto the carb inlets.
Then have some more fun routing the four drain tubes and two vent tubes between the engine and swing arm !

Get a heat gun on it which should help separate the two.
That plus the persuader got mine apart, although I don't recall that much bother.
Plenty of ACF50 (grease) on the two surfaces before re-assembly as well.

A well preserved set of OE headers, collector and silencer is a sound investment. 

To be used, the engine would need , at the very least, a top end rebuild.
Even with oil in the sump, the internals, bores, rings, valves  etc would need inspection for corrosion.
The exhaust will also have some degree of internal tin worm after 40 years.
As a collectors dream or a museum piece maybe.