As a matter of fact, I have flown something very similar...with a Saito 91 4 stroke. We ended up adding quite a bit of lead (12 or 14 oz if I remember...) to the firewall to compensate for the lighter engine. The cub flew just fine with that engine, although going up was not its' forte. I was able to do loops, hammerheads, wingovers, even inverted flight with that engine. But I have to be honest, it was not very exciting to fly. I swapped that engine and the ballast for a G-20Ei and it is much more energetic, and fun...
Normal power for these birds is a 120 4 stroke, or a G-23 / 26......but there is flexibility in both directions..