I have a civilian GP Ryan STA myself. It's powered with an OS 120III w/pump and a 16x8 prop. I have additional weight in the form of the Robart struts and an MGA pilot (I don't have a good weight scale yet, but two cheap ones each put me at just under 12lbs). Regardless, it is definitely not underpowered.

However, it is also not overpowered either. I can't fly that thing around like a lightweight aerobat, but I do get off the ground quickly and get very good scale response (actually, I'm sure it's much greater than scale). I have to nurse it through some manuvers, paying attention to my airspeed, but I'm enjoying that tremedously in a scale-like way. If you want to fly specifically on the prop, then it may not be enough motor. But then, that might not be the right plane for that either. I cannot just point it anywhere in the sky, at any time, and have it go forever in that direction, but I've decided I don't want to do that with this plane.

I guess your engine choice depends on how you want to fly it. I have to admit that the cool checker pattern sort of begs me for more engine, but I'm satisfied with what I have. BTW, if I'm not mistaken, a YS 120 has at least one more horsepower than an OS. I'm not sure they're a good comparison against each other.

I've debated for a little while now about putting more engine on it, but I've decided to keep this nice flyer as is. It's an awesome platform for imagining I'm flying something full scale and for practicing fully coordinated manuvers. I'm kind of new at all this, but later on I'll get a proper aerobat for completely unlimited flight.

Hope this helps.