Mine got its 3rd powerplant change (I love experimenting). It started with the O.S. 40FP then went to the thunder tiger .46 pro and now it has a older O.S. .61-FS four stroke. For this engine I also had to move the battery behind the CG. To be honest the .61 four stroke with a 12x6 prop has performance inbetween the .40 and the .46. The .46 definately had more power to weight ratio and pulled it verticle better. BUT the .61-FS isn't that far behind and has that beatiful 4-stroke sound. I bet the .70 surpass is the perfect engine for this plane. I think my .61 is about the same weight as the .70 but about 3/4 of the power.