i love it when people ask me how to make a honda 200 go faster. the answer i give is buy a two stroke. xr200's are a great bike. ive got an '01 and is stock. i race road bikes round ruapuna and have a gl145 bucket racer. now all models of honda are different, all of them. an atc200 has low comp head and piston cam and other shit.xl and xr are two different engines al together as your probley awear. now you got the flat slide but it wont work untill hi rev range. for this you need the yoshi cam grind. mark my words, you will thank me for this!. slightly heavy springs and porting. dont go bigger on the inlet side, but go crazy on the exhaust. hondas love to breath.good luck with the whole project man, but remember its still only an xr, and a dirt bike. if you make it a rocket it will be hard to ride , and if you hot it up to much i have no dought it will bend a valve some where in the arsehole of no where and you'll be walking along way.keep it simple just like mr honda did........