BME
3W
DA

Above in no specific order.

Pick one of the above by whatever method you choose and you won't make a bad choice. All are powerful. I'm running a BME-50 on a pig of an airframe and it just jerks it off the ground. I have run 3W's for competetion but haven't had anything in the 50cc or so class since selling my 60cc a few years ago. Last I heard it is still flying. Have a DA on order for another plane.

Interesting comment on the Brison listed above. I'm helping a freind that is installing a 90cc Brison in a 33% CAP. Brison highly recommends a Hyde soft mount (really a good product) however adds considerably to the cost and weight. By thei time he gets done with the 90 single and soft mount it will be really close to the cost of a nice twin. I don't personally like the mechanical spark advanve they use as I had problems with one on another brand of engine a few years back (just wouldn't stay in adjustment or broke linkage parts).

EXCAP232