The four stroke swings a bigger prop at slower rpms and when throttled back will give the "womp" sound similar to a real aircraft engine. Unless you have an unbalanced prop (which you do not want), it is impossible to make a two stroke try to sound like a radial.

With my air intake open, flaps down, and throttle at 35%, I can make a realistic sounding flyby with a OS 1.20 4 cycl. powered TF P-51.