OS 1.20 4 cycle will be sufficient, but I still don't think you'l be happy with that combo in the long run. It will be good until you get comfortable with the stearman, but you'll be sweating for more power at times. It will just barely be enough to pull it through a loop. The experience I had with the stearman had a YS 1.20 on it, and I still thought it was not enough.

Four strokes are not always equivalent to realistic sound. That is an expensive choice if that is what your going for. If you go with the bigger 2 strokes, you have a deeper, or more throaty sound ( with a pitts muffler), you'll be using less throttle because you won't need it, you'll be swinging the bigger -more scale-prop, it will cost you less money, and you'll have the reserve power you'll want, as opposed to the OS 1.20 4 stroke.

Don't get me wrong, OS 4 Cycles are wonderful engines, nice and strong. But it will simply be marginal for this stearman.

I have seen people have good experiences with the OS 1.60 twin.