I've never really researched that... I seem to remember the cylinder head being different from the aircraft FS40 to the FS40S-C... When in doubt, look in the manuals and compare part numbers. They don't lie - if a certain part number is the same on both engines, it is the same part. The block is likely the same, except for the color. The crank, cam, valve springs and spring seats will be different, but other than that, I'm not sure.

I have no experience with the 40... I know the 52 and Alpha 56 responded well to a 3 needle slide carb, but I don't think a smaller displacement engine would work the same with those larger carbs. Playing the carb game is kinda like roulette. You never really know how well one is going to work until you try it, and try it with all possible fuel system setups - from no pressure to pumped/regulated. Every carb responds differently to each situation. I would start out by trying a small-bore .21 carb, or possibly a big-bore TZ-18 carb, or maybe a Traxxas 3.3 carb.

I don't currently have a flywheel for the 40. My .26 flywheel would probably be too small.. Your best bet is to have someone with a metal lathe make a steel flywheel for you that's the right size for your application. That same guy would likely have a drill press to make the clutch pin holes, and be able to press in the pins. The alternative is to adapt a .21 flywheel to fit and have a steel sleeve pressed or bolted to the outside of it to bring the overall diameter and weight up to what you need.