First off exhaust's get a big
from me, on a race bike (unless its a wr or crfx, those come plugged up and even then you dont have to replace the pipe) offer little performance gain, are very expensive, and some will make the bike obnoxiously loud, not good for keeping riding areas open.
For a wr I would recommend the GYTR muffler insert, it will keep the bike fairly quiet and will not cost you much, if anything in performance.
Now onto general mods.
A great place to start is with the carb, the power now is a great mod for the money, and a JD jet kit will give you spot on jetting equaling good performance
F2 Racing also makes a kit for this called the power back (not sure will have too look it up)
If you have a trail bike version you probably have an airbox lid, trash it.
In the engine I would start with a set of hotcams, they will help unleash the beast!!!! reports are of a higher reving engine, that gets there much quicker.
You may also want to look into a high compression piston, many including myself like the idea of 13.5:1, I would highly recommend one from wiseco. for reliablity, you may want to go with a set of Kibblewhite Stainless Steel valves, they offer improved valve life, but beware as they also put more strain on the timing chain and the motor will rev slower.
Ignition mapping can make a good improvement, an aftermarket cdi box will result in being able to reach higher rpm's thus tapping more of a power range from your engine.
Suspension, really a plus on any bike, getting the supsension set up for your weight and skill level will make the bike the handle better, and it will probably save you from a few falls.
This is just a general guide, I hope you find it useful.