My daughter just came off a 2001 PW80. She raced that bike for about a year in our local playbike class and consistently beat all other PW's, TTR90's, XR80's, and some XR100's. Simplest and quickest way to make a PW80 go better: Pull the washer out of the head/pipe joint and throw it away. Remove the snorkel from the airbox. Replace the stock filter foam with a good quality aftermarket foam, you will have to cut it yourself. Open up the airbox, I don't suggest drilling the front as mud will quickly plug it, but drill several 1/4 inch holes in the back along the sides, make sure you don't bypass the airfilter. The PW's run very rich from the factory, after all this you will be much closer, I live at 4500ft and from this point all I had to do was drop the needle one clip. If you really want more power, replace the ignition with a pre-1994 PW80 ignition, this will allow you to advance the timing. Also, if they race, remove the silencer. The bike won't be much louder, but you will gain about 2 hp. Don't trail ride without it though. If your kid rides it hard and tends to like a bit of airtime, frequently check the engine mount bolts and the swingarm bolts, they will tend to loosen up rather quickly.
Eric B.