I had ( briefly ) a waco bipe that was slightly smaller ( 60" top wing span ) and it flew well with a 91 four stroke. A 120 would probably work well for you. I'm putting together a 72" tiger moth bipe. The vendor says it flies on a 70 fs im going for a 91 allthough with a bit of surgery I could cram in a 120. With a 120 Id have plenty of power. Anyway the waco flew at at a liesurly pace at 1/2 throttle, more gas for acrobatic moves.

If size is an issue and cash isnt check out YS engines. The 110 is a 91 size case, the 140 is a 120 size cas. Both are very powerful for their size. They are supercharged and have intregral fuel pumps.