i took apart the carb and cleaned it out real good. sometimes there is a glaze on all the carb parts. i get rid of that with a cleaner. brake cleaner works good.
the exhaust is critical mine was clogged, so i hit it with a hammer (not hard enough to dent it) and let the loose stuff fall out and blasted it with compressed air.
buckaroo bob did a cool thing with his exhaust, he added a expansion chamber.
i also took off my head and cleand it with a wire brush. i then shaved a little off the top of the cylinder to give it better compresion. this is machine work, but i did it with a grinder and a flat file. if you are not experienced i would not suggest doing it this way, for i have messed a couple up in my early day's
i also boered a little bit out of the carb. if you do this do not mess with the needles. the object is to allow the motor to suck more air.
i dont run an air filter. this is contraversial, but i run a screen to keep out the big stuff.
the electrical system is very important, though i have not messed with mine yet.
my freind is running a coil from a car. it works great,i'll see how he rigged it up.
i did see replacement exausts somewhere on the web. i think this may be a large part of your problem.
another thing that slows people down is simply improper brake adjustment and bad bearings. i know this sounds silly, but i have worked on many bikes where the problem was just misadjusted brakes.