If you want real performance then it is not cheap but is out there and the combination is an OS .61FX with an Ultra Thrust resonent or quarter wave tuned pipe Here:

http://www.pspec.com/details.asp?ProdID=61&category=8

The engine is around a 180 bucks and the pipe seventy Yes expensive but in my opinion it will outperform all the engines mentioned so far in this thread and provide superior throttle response while your at it.

John