+1 for K&N filter and an aftermarket exhaust. They will make it breathe better and you will feel the revs pick up a little quicker. The real benefit is the great noise the engine makes when you give it some. The aftermarket exhaust will also shed some weight. This is where my 'performance' tweaking advice stops for a Wee.
If you want to spend money on the bike, spend it on suspension and brakes. Get a fork brace, fork springs, rear shock spring, EBC HH brake pads for the front and braided brake hoses for the front. These are all relatively cheap mods, but they will transform how your bike rides, handles and stops. These changes will be more valuable and, more importantly, more noticeable than tuning your engine.