Slightly underpowered plane, too small wheels, inexperienced pilot= a bad combination. I doubt you tip stalled just because Avistars have pretty stall resistant wings that are designed to root stall first. What is far more likely is either you took off too slow which got you into a regular old stall that then let the plane be rolled by the engine torque, or your wheel caught a bump causing you to yaw a bit just as you took off which, at that slow speed, could set up a spin that you didn't have the flying skills to recognize and get out of.

You're on the right track for fixing the problem. Bigger wheels will definitely help, as will more power. You don't need a 55 AX though. If you're planning another plane that needs that engine down the road then it won't hurt anything to use it, but you're currently using one of the most anemic .40 size engines available. Moving to any ball bearing .46 will give you a 25-30% power increase, which will let you do the gun and jerk takeoffs that you probably need to do on your rough runway.