For about the price of a .55 AX you could just go ahead and buy a Thunder Tiger F-75s, they're $169.99 with free shipping at
http://www.thundertiger4u.com right now. If you'd rather save some cash and stick with a fast, reliable two-stroke then give the Thunder Tiger Pro .46 a try. It will fly a Big Stik .40 ARF with plenty of power on tap.
If neither of these choices is expensive enough, then you should order an Enya 50CX from
http://www.enya-engine.com Enya's are wonderful engines that are very powerful and very reliable. The 50CX will set you back about $215 plus shipping.