Weigh the plane and let us know. My Reactor originally had an older OS50FSR with a Mac's tuned pipe. It weighed approx 5lb 10 oz with that setup. Best prop was a 12x4 Xoar, but even then, it wouldn't hold a hover for long. From all accounts, the 55AX is a fair bit stronger.

I switched to a Saito 82 with a 14x4W and had plenty of power for hover. All up weight was almost identicial, as I lost the weight of the tuned pipe setup. If your plane is over 6lbs, you're asking too much of the 55AX. A 4 stroke .91 would be good; you're going to have to juggle setup (battery location, etc) to keep if from being too nose heavy.