The Avistar is a great little plane, it is capable of aerobatics as well as training without any modifications. You will need some more power than that LA 40 though. I have taught students to fly on differant kinds of planes and I think the Avistar is best. I would definetly say the O.S. AX 55 as your upgrade. I have flown the Avistar with this engine and she flies real nice, plus you will need the extra power for the Decathlon as they are a little dicey with that short fuselage  taxiing down the runway.  Good luck, hth    John