I second the nomination for the Thunder Tiger .46 Pro. I'm flying one now in an LT-40 (I really needed the extra weight up front). A search of these forums or of the web/usenet will turn up some pretty solid references to this particular engine. It's well made, has plenty of power and is available at a bargain price.

FWIW, mine seemed to enjoy a little more nitro to idle reliably (10-15%). For a draggy trainer type aircraft, I'm swinging an APC 11x5 prop.