Start with a boyeson power reed, It will yield a good power increase but you will have to adjust the carb to before it will run right. I think you have to move the clip down one, and I know you have to adjust the mixture screw. PWs run rich out of the box so the main isn't a huge deal, if you do feel the need to experiment, lean it out by one size. number on the jet -2.5. I don't recall the size of the main right now. If you won't be running in the woods, pull the silencer out of the exhaust and put it in a box. This also make it sound a lot better. Install a better aitr filter, maybe drill a few strategic holes in the airbox. If you still want more, it can be opened up to 60cc, a rev plate can be installed, and exhaust porting helps. If he will be riding hard, I recommend replaceing the crappy rubber footpegs with serrated ones so his feet don't slip off the pegs. This is a safety thing. I always ran motul 600 in the injector so that the exhaust port stays fairly clean. It runs about $8 per qt, but I didn't use a lot of it. You can adjust the pump to the minimum spec to gain a little more power. A lot of people disable the pump altogether and just run premix. I did not. Oh, FMF does make a fatty pipe and turbinecore silencer for this little machine. If you will be racing, most of these suggestions are not AMA legal so you should consult your local referee or risk DQ.
peeweecycle.com is a good shop to get stuff from, I have dealt with them on more than on occasion.