I've built and flown three Kadet Seniors and put .60 2 strokes in all of them. Part of the reasoning for this is I fly at 5000' ASL. A .71 4 stroke would be adequate for almost any altitude. The .91 will not make a hot rod out of the thing but you should be prepared to add tail weight. With the exception of the P-51 not many aircraft can be improved by over-powering. You should also consider adding some shear webbing to all wing spars and vertical/horizontal bracing to the tail feathers. A conversion to taildragger status will increase the fun too.