Air filter and exhaust
T-4, HMF, or yosh
If you really want to see a big difference get a dual 35 FCR carb kit. You won't make good clean power until you get one of these. It will turn that punk 660 into a real 660 and you won't have to deal with those stupid vaccum operated carbs. They are crap. I'm not talking out of my ***** my cousin was so fed up with them he bought these carbs. Installed them and made a huge difference, next going in is the hot cams, cp piston, and corrillo rod he just bought and then he's going to port that beast.
TRX450R...Pulse Charger, 12.25 HH, HotCam S2, KW valves and springs, P&P, UNI filter, airbox outterwears, Woodward fuel screw, Denton stabilizer,ProArmor K/S, Elka Elite longtravel 17" w/dual linkage, Figair airbrushed hood, Holeshot fronts, Razr II's rears, AC Propeg nerfs, +1 1/2 stem, Armidillo skids.
And a very well modded 03 warrior, engine, suspension, etc