Jim,
I'm using the 180 Saito and it has a lot of power. There are times when I go off course when I'm taking off so I just give it full throttle and it jumps right off the runway. I never had a 120 or 150 in it so I can't really give you a comparison.
On the other hand the Stearman isn't really supposed to be a speed demon. I think the 120 would give you more of scale flight.
So if you really want some speed and your thinking about the 150 you might as well use the 180 and really give it some punch. The 180 fits in with no problem.
I used the stock muffler and it works fine. I have it exhausting out the bottom with an extension. The only problem I had with the muffler is it has a tendency to loosen. I made a bracket to keep it from rotating loose and I also use a little high tempiture Loctite on the treads. Make sure you don’t get any Loctite in the engine.
Good Luck and Happy Landings
Shubova
[8D]