I'm surprised no one has answered you yet. There were several active threads on the GP Chipmunk a little while back. (Do a search) I guess since you didn't mention the Chipmunk in the title, they haven't clicked in.

Anyway to answer your questions (some of them anyway), in spite of the similar wingspans, the Sportster is a much larger and sturdier airplane. It is designed to handle the extra power and absorb the stronger vibration of a gasoline engine. The Chipmunk has a brittle fiberglass fuselage and light duty construction that wouldn't last very long behind a 40cc gasser.

Oh yeah, the Chipmunk cowl is very tight. If you put that gasser in there, you wouldn't have any cowl left.

Jim