The GP streaman REQUIRES: Engine: .91 (14.9 cc) - 1.20 (19.66cc) 2 or 4 stroke
Depending on how you want the plane to preform then choose the apporiate engine. I prefer scale to alittle over powered depending on the aircraft. The Saito 100 is a light motor.
A new one at our field turns a 15x6 at 9k. Less power than most 120 four strokes and lightness is not needed in the front of a Stearman.

Carl