I would recommend you go for a piped 61 of some kind, most probably an os 61 fx with an os pipe to suit, you wont go wrong with that option.

I have found that when it comes to engines you don`t wont to compromise on quality and reliability, espcially for aerobatics. I myself have just finished putting together a Zen 120 and have done extensive modifications on it, i have fitted an OS 140 RX, their was a fair deal of playing around to fit this engine, but its light reliable and will fly vertically out of site on 3/4 power.

Some pics