At the very most a 70 four stroke, but still might be too much. A buddy of mine had an O.S. 70FS Surpass on his and it flew nicely at 1/4 throttle. Thing is, that plane builds tail heavy so the extra weight will be needed up front. If you're going with a FS I'd recommend a 4 or 6 pitch prop. Very nice flying plane. A real floater on landings.

Good luck.