you don't bore cylinder heads, you bore cylinders out to accept larger pistons.. jetting and exhaust will net you almost nothing without some work on the heads like porting/polishing to match it...but again, the gain is so small for the price it's not really worth it..especially on a street bike, or unless you plan to dig right into the motor and go all out.
easy simple bolt on things to do are a 650 cam, you probably really notice it when get into the high RPM's.
i am not sure if you are just asking to have more HP over all or if you really mean you want the bike to have more oomph at low rpm's.... getting more low end torque out of a small cc 4 is just not gonna happen, it's the way flat plane 4's are. RPM to make hp. You want massive low end torque, you need a single or a twin to do it, but you trade off high speed. (although not so much anymore, my 999 hits the limiter at 11,000...thats real high for a twin)
mlinder said it best...
it can be a trade off...you can go to town on motor work and make one of these perform like crazy, but you will always need to rev it. It will make it almost undrivable in the city. My friend had an 05 Kwaka ZX10 and put in a set of cams that he did no research on and he pretty much lost all midrange. So the bike was only ever making good power when it was bouncing off of the limiter. He sold it..
