An exhaust by itself won't give you very much bang for the buck. If you go with a 19mm carb, and a pipe you can add 2ish HP, and 5-7 mph.
To really feel that power gain CVT work is almost mandatory. Rollers balanced to the new set up, and a tune-able clutch + a control spring make a world of difference.
I look at tuning like a two phase deal. the logical next step would be a bore kit. It can add about the same amount of power, and MPH, and that would be the time to polish up the cvt. A good bore kit and clutch should go for approx 200.00 as well
Check out SPI
http://www.spi-parts.com We have a arrangement w/ the owner to provide dealer pricing on performance parts for Kymco Forum members. I'm sure he can set you up with a proper pipe/carb/roller set within your budget.
Louie Peverini
Scooter Parts International
800-591-2670
Check out the site but he never has EVERYTHING listed so phoning him is a better choice. The guy is a world class bike mechanic,(team Yamaha pro) and will help you out.
Post up, and let us know how you make out. I can help with any install questions.