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Impressive but wonder why they haven't carried this on?

Compression ratio - I agree with the very high compression ratio approach for this project. Say 15:1 if you can actually achieve it. As has already been stated with small throttle/openings the effective comp Pressure will be comparable to say 9 or 10:1. I reckon this will be the biggest single improvement but you'll have to be disciplined not to use full throttle or risk severe detonation. High octane pump fuel.

Carb - small (1" ~ 1.25") SU type carb as these are very fuel efficient.

Exhaust - extractors, wrapped for heat retention/gas speed, and with std bore outlet (not 2" as this will be too large to maintain gas speed at low rpm/low throttle openings.

Camshaft - std cam (if not an even milder design).

Head - round port head with the most efficient high swirl combustion chamber design (maybe A12 roundport head?).

Hot air - I'm not entirely convinced on the hot air for economy vs cold air for power theory but it would be easy to experiment with this once the rest of the engine is set up.

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Can't see an issue with a thin bonnet after all it's just to keep the rain off.

Thin = less weight.
Weight = "the enemy"

Seems to be a common misconception that sidedrafts somehow go out of tune over time.

Once they're set up properly (Dyno to ensure correct jets etc) there's not much to go out of tune unless the idle mixture or balance is fiddled with or engine spec is changed.

A Dyno is not necessary to re-set idle mixture and balance - just need someone who knows what they're doing.

Could be running too rich? Very dirty aircleaner maybe? Choke stuck on? Idle mixture too rich? Incorrectly jetted carb/s (if non standard)...

As alluded by Cheater, the carriers are interchangeble between boxes.

Lengths (throw) varies to suit Flywheel thickness (bigger engines have thicker flywheels as standard at least). Forks also differ.

Suck it and see!

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I think brass "K Liners" are better (may be a proprietry name but I'd say any good machine shop will know what I'm taking about). Take head to machine shop and they ream out the existing guides and put these liners in. Better heat transfer and brass has self lubricating properties.

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Assuming the engines been opened up, just measure the stroke of the piston. Should be either 70mm or 77mm (or maybe 82mm).

Damaged one of the inlet valves on may A13 race engine last outing.

Apparently the existing ones are Volkswagen but don't know what model/engine.

They are 40mm, Stainless Steel, and have waisted stems/tuliped.

If I can't find out what they are, what valves have others used which would suit?

Thanks