dude, listen to me. Take the money you are thinking about spending on big expensive performance mods and replace all the worn out parts on your bike. For example Tire, chain and sprockets, air filters, grips, brake pads, and so on. Do not use expencive vp racing fuels because your stock engine does not need it. Use all the extra money on gas for the bike, gas for your truck to get to riding spots. Ride as much as possible! Improving your riding skills will help you out at the track much more than spending alot of money on engine mods. Oh and by the way I had an 02 cr125. I rode it in stock form, then again with cylinder porting/134cc big bore. The 134 big bore was a noticably faster bike but nothing night and day. The 250f's are still faster, and much easier to ride because they have more low end power. Another thing to note, I also had both the front and rear suspention revalved. The suspention work seemed to help out on the track more so than the engine work. So in conclusion, Save the money you were going to spend on performance mods and use the money more wisely. Just ride alot and improve your riding skills. Dont have the mentality that performance mods will make night and day faster on the track because it wont. Only hard work practicing! :) And.....I am no expert by any means!
Last thing, If you do decide to do engine work, I hear great things about eric gorr.