I have flown a 35 lbs pup with a 3.7 Sachs mag engine with a 24x10 prop and it flew great - capable of rolling circles and lots of other aerobatics........altitude here in Indiana is about 750 feet above sea level.

I also flew a 37 lbs pup with a G 62 (mag) also w/24x10 prop....also flew great, but not quite as strong as the other one. I go to a fly in in the Ozarks where the altitude is about 1700 feet and the power is affected somewhat, but it doesn't struggle at all. I also flew this same pup with a Zenoah 445 (74cc) twin (same prop). I liked the extra power a lot. For scale flying a G62 or equivilent (DLE 55) is fine, but for spirited dog fighting a bigger engine is loads more fun.

I say go for the extra power, up to an 80 twin or an 80 or 85 single - but would not recommend a 100 cc twin or larger. Just my 2 cents worth......Art