Im with flatbeddennis, the engine and the plane are not the problem.
I fly a Hobbico Superstar with an O.S .46fx, My skills are still very basic but I have done dives and pull ups without any problem. One thing is to make sure that when you enter a dive that you reduce your throttle to almost nothing. When you pull out do so SLOWLY adding elevator.
If you slam the plane around then you are of course going to have problems. Trainers are not designed to be aerobatic planes.

While the avistar does have an semi-symetrical wing if you slam it around it will break.
I watched a Avistar fly this past weekend, the guy climbed the thing almost out of site. Then proceeded to enter a dive and came fairly low in my opinion. He pulled out with a smoothe approach and had no problems whatsoever. He did actually slam the plane a little more than what I thought was normal as I watched him fly.
He had no problems though.

The Avistar is a great plane and can handle plenty with a decent flying style. The O.S .46FX is quite perfect for these trainers in my opinion, however some may feel more comfortable with the 40LA.

Dbow