Believe it or not back in the day four stroke tech used to build a 500 kit for the warrior and it was a ******* monster!!! ;D ;D ;D They stopped making it due to lack of sales probably because it was expensive as hell. Today powroll makes a 437 kit as the most extreme I've seen, but for the best reliability I'd go with the alba 400. I wouldn't let them do the head work though, I'd rather send the head elsewhere to get a better port job, cam, and larger valves. I would much rather them just do the crank and cylinder work on the motor, you can crank out much more hp than their full setup.