The TT 46 Pro is the best performance fit & buy for the Avistar -- the SX is a great engine, but it's too expensive for an engine of that size & output, & it is actually more powerfull than the Avistar can comfortably deal with (so is the TT for that matter). If you are very brave, for barely more than the price of the TT Pro you can put a GMS .76 on the Avistar -- that would be a highly entertaining experience that the rest of us would like to hear about.