I'm going to mount a OS 61FX on the s40 I picked up at a swap meet/auction last fall. I saw a video on a flying giants form where the guy had a OS 50 on it and it seemed to be plenty of power to clime out of a hover pretty fast. From what I have research on this plane a 50 two stroke is plenty of power as well as a 70 four stroke. Thats not to say that you couldn't use something bigger like some of the other guy's are saying they are using.