I would be prone to use the OS FSR and a low pitch prop if you want the Kadet to do <u>some</u> sport aerobatics. I'm currently flying my Kadet (the kit with barndoor aeilerons added) with a Magnum XLS 46 and it is a good slow Sunday flyer. It can do a snap loop (hammer full up) but not a big open round loop: not enough thrust.  It will do rolls and fly inverted (with a good deal of down elevator because of the lifting wing). Haven't tried knife edge yet.

I would not consider the Fox or the Enya as they are heavier engines and the probably necessary added tail weight would negate some of the benefits of the added power. Of course, if your building the kit, you could shorten the nose to compensate for the added weight for any of the larger engines.