The factory restrictors are the jumper and the throttle screw.
If you are want you unit to faster then 30 (or so) faster, then you'll have to start applying mods. These mods range from engine, pipe, tranny, carb, chain/sprockets, shocks and even wheel sizes. To obtain more details, you want want to surf / contact the following companies:
http://www.ebay.com/ -> search for ATV * Good for things you know will work with your package. For example, K&D Filters, etc.
During your conversations with each of their respective Modification Consultants &/or Sales Reps, ensure you specify your riding conditions (re: racing only, trails only, big jumps, mixed, etc.) and if there are any limitations you must stay within. If you race your quad, some modifications are NOT allowed (depending upon that specific track). For prices, you can "shop around" or send each into discounter counter offers. Each may or may not lower their prices for you. As a final note, try to stay with the same "bundled package" of modifications from the same company. For example, don't mix clutch upgrades from one company, pipe upgrades from another company, and head upgrades from a different company. Try to stay with one company for all "specialized" bundled packages.
I hope this helps...