Well, I'm not sure what you consider expensive, but you could look into getting a set of pipes for it, which would definately boost your hp. You'd also have to rejet your carbs. Also a set of power domes would give you an increase. A clutch kit would be nice. It doesn't boost hp, but it does allow you to utilize the hp you have better. If you did all this, you'd definately need to pick your track as well. Just some ideas..........
1997 XLT SP 600

1997 XLT SP 600