You've already addressed a lot of these issues, but here a few things that I've personally experianced that can cause overheating:
Poor Air Flow! Sounds like you've already address a lot of air flow issues, but like Anthony said, make sure the flaps are opening. Even if the thermostat has been removed, you need the flaps! If they are not there, that could be part of the problem . . . lots of folks remove them thinking they restrict air flow, but they actually "guide" the air flow to the hottest parts of the engine.
I had an engine once that had the deflector tin that goes between the cylinders installed on the top instead of the bottom. Look and make sure it's installed underneath (should be on the bottom of the cylinders up above the push rod tubes). If it's not there, that's bad . . . if it's on top, that's REAL bad!
I've seen the generator pully spinning on the generator shaft because of a missing or sheered key, and I've seen the fan spinning on the shaft due to a loose nut on the back side of the generator.
Make sure all the tin is in place! On a doghouse style oil cooler it's way to common to see the lower piece of tin missing and letting the hot air off of the oil cooler come back into the engine compartment.
Make sure the fresh air hoses to the heater boxes are hooked up. If you aren't using heater boxes, plug the outlets on the fan housing.
Make sure the rear engine compartment seal is in place. Without it you'll suck hot air from under the engine right back inside the engine compartment.
If all the air flow issues look ok, next is timing and valve adjustment. Too much spark advance will melt down an air cooled engine. If the valves are too tight, on top of effecting compression, the valves won't make as good of contact with the valve seats as it should and will not allow the heat to be transfered to the head as quickly . . . I don't think this will cause the entire engine to overheat, but it can cause the exhaust valves to fail prematurley.
After that, about the only other thing I can think of is a lean fuel mixture . . . common causes of a lean mixture are a worn throddle shaft in the carb, bad boots on the joints of a dual port intake (VERY COMMON), or a bad intake to head seal.
Edited By Zen on 1031691373
