Your best bet is to buy your pipe used and your engine parts like pistons and valves new.
Remember when modding to have a goal or style in mind so you get parts that compliment each other. Like you don't want a huge carb like a 39mm fcr for a low rpm putt around a golf course build and you won't want a 36mm Btm carb for a all out build. You must keep your performance style in mind when choosing parts.
Like my setup is on the more radical less common end of the spectrum 12:1 comp piston custom larger diameter exhaust high lift cam, fugue carb and no air box. While most woods bikes use a moderate carb short lift long duration cam and smaller exhaust for torque.
Just keep in mind your style and terrain when choosing your performance mods. Like James said every mod should compliment the others