I'd happily take the "worst" engine mentioned so far, whatever that might be. The Super Tigre G-40 and G-45 both have great reputations for power and reliability. The Thunder Tiger Pro .46 is a fantastic engine. O.S. Max also makes great 2-strokes in the .40~.46 size range.

I'd probably pick the TT Pro .46 myself, but the G-45 ABC would a really great alternative. I might lean toward the new G-45 if I couldn't pick up a used Pro .46 for the same or less money. It's nice to have a warranty and to know you're not buying somebody else's problem.