My bro has a 99 400 4x4, and it is all stock. It has plenty of power. The 97 and 98 models are hard to get aftermarket performance parts for.

If you look around, you can get aftermarket pipes and filters for it. They will help.

You can get a secondary spring for it from EPI to help with bigger tires. You might be able to get other clutch stuff too. Call them, 218 829 6036

Mike Penland may have raced that model, I don't know for sure. He may have some tips. Mike Penland 706-746-2812 (mikes bikes). He is a good guy and will be happy to talk to you.

Four stroke tech has (or used to have) a 500cc kit for the Prairie 400. That will do more than all other things you can think of combined. He may have a cam. I know he can do head work. You might be able to beat a whole bunch of machines with his help. email mdunlap@bedford.net or call (814)842-6159.