To be a bit different in response to your questions I have actually had a couple of Avistars with LA engines as well as ST and OS 40/46's. An Avistar is not exactly a trainer because of it's semi-symetrical wing. While it can be used as a trainer it is a lot better plane for simple aerobatics than a flat bottom wing trainer. All those I have seen were Semi-symetrical but it's always possible they made a flat bottom wing version.
I flew one for a long time with a Thunder Tiger 46 which is a very good engine(not to be confused with a Super Tigre). Your problem is going to be weight. The avistars I have had easily balanced out without any extra lead using an LA engine, so if you use a ballbearing 40/46 you may need quite a bit of weight on the tail end to balance it. The OS55 is an even better engine but does weigh just a little more than most 46's.
I have flown two different 40 size Decathalons and both were a real handfull on the ground. You might do some research on them and get advice from those who have actually had the plane. Be carefull using a two stroke on a plane that you plan to invert the engine. 4 strokes work well that way but two strokes can be problematic no matter how well they are tuned. The Decathalons I flew had side mounted engines.
Fred