Since you got it used, there is a good chance you have the older engine. If so, anything you do to get more air to pass through the engine will give you startling power gains. If you have the later 865CC engine, nothing will do much until you change the cams unless you intend to add a turbocharger. The exhaust valve timing on the later engine is terrible for other sorts of power mods, but ideal for supercharging.
Once you've done all else, if you want to have even more power, you can get big bore kits. You can go up to about 904CC with bolt on kits. There are a few bikes bored to over 1100CC, but you have to modify the crankcase bores for that.
