how fast are they?
I don't know, I really haven't rode them yet. I am waiting for my son to turn 15 and get a legal moped license. He will turn 15 this January, so we will be riding next year. They have a BB cylinder kits, bigger carbs, large intake and reeds, and a larger final drive gear for more speed. I am using the stock pipes to keep the noise down.
Some guy on Ebay sells the kits. I would guess that they would do around 40+ mph or so, but I am just guessing. I built them both last winter when the ice never froze and left me with nothing to do... We live in town so could ride them from the house or on vacation.