what everyone else said..
advanced timing
porting for where you choose mose likely mx or agressive mx somethin like that
pipes-(pro circuits are great all around, fmf fattys, dmcs, paul turners, lrds) anything but cpi or shearer pretty much lol
CARBS AND FILTERS I see it time and time again these make or break a bike.. The wrong carbs make things hell I like pwk 28s because they work really good all around. I have a 2 into 1 its alright but I miss the topend pull of my pwks.
-1 swingarm mabey?
Skids?
TIRES! Get good tires personally I like a nice normal turf tamer.. Klaws are good knarlys, razrs, ohtsus are bad ass
suspension (spend the money get something quality)
Cooling system!!!! Cool heads, billet impellers, good fluid anything to keep it cool in tight trails
And after all of the above just remember to keep it jetted rights and play with gearing and buy a shit load of bling and stickers because we know that makes every bike faster of course!