I have the Saito 150 that I recovered from my crashed Yak.
I was considering using a 120 I have but the Saito 150 weighs the same as the 120 and is lighter than the OS 120FSby 2 oz.
I will not use a 2-stroke inverted so the 150 has worked out perfectly.
Garcfield is correct about the power. On several pattern maneuvers, I need all the power I have. You need to learn how to manage your throttle between passes to conserve fuel.
What we are saying is don't under power a pattern plane.

I originally had an OS 46 on my Venus 40 but the Os 50 was much better and the 55 would be great.