RE: Which 4 stroke engine do you recommend?
Might try a gas conversion of a 4-stroke. Lots of folk go with on board glow igniters & for similar expense a RcXel ignition can be purchased. In many cases glow carb can be used with gas, just becomes very sensitive - a little tricky to adjust. In addition to less mess & greater reliability, it uses a lot less fuel. there are some good threads on this site concerning 4-stroke conversions.
Mike Brennan
Club Saito #723