I teach a lot of people to fly and currently have 3 Avistars that I use for this purpose. Two of them still have the Os .40 La and have plenty of power for basic flying if tuned properly with the right prop. But for aerobatics and performance, I put a Magnum .46 xls on one of my Avistars. It has all the speed and power this airframe needs and idles better than my Os .46 ax and fx engines on other planes. It has never deadsticked unless I wanted it to. I also took the dihedral out of the wings, made a larger rudder, and set my CG at the aft limits of the reccomended range.
Taking the dihedral out had a minimal benefit for inverted flight, but there is not much dihedral in the stock wing anyway, so you would be fine with a stock wing.
Enlarging the rudder made Knife edges easier to perform, but it still won't hold altitude forever in a knife edge, a .55 ax probably would help this, but would be harder to get the Cg aft.
Moving the Cg aft allowed a slower landing and more responsive control, but is still a very stable flyer. To get the Cg aft, I had to put the battery way back in the fuse and put the rx as far back as it would go. Hope this helps, and hope you enjoy the Avistar as much as I have!