Well there are a few, but allot depends on what you are going to use it for.

If speed is what you want then look at the Webra 40Q, Nelson and Jett 40's.

If it's general performance you may want to look at the GMS 47, rather in-expensive and produces goobs of power, the TT46 - a good, strong engine without many problems, also look at the Super Tiger and OS.

But the real thing you have to determine is what application you are going to use it on, this will help decide what engine you are going to get.