I bought a Yamaha QT50!!!!! I love it. It is so much fun to drive and I get as many smiles and waves as when I drove the mechanical nightmare that was my weedeater bike.
Now my plan for it. I plan to put on the right swing arm from a MJ50 (towny) and maybe it's two speed crankcase because mine is only one speed. I also plan to put on a Malossi 70cc big bore kit, 40lbs clutch springs, bigger carb jets, a Yamaha Sting exhaust (if I can find one) and run dual off road shocks which I will be able to do since I will then have two swing arms. All of this said in done will raise the top speed from 30mph to around 60mph (yes I am a lunatic).
I bought it for $320 and did a little carb work and now it runs great. Some guy told me it is worth $700-$800 I don't know if he was right or not though.
I also love that parts are so available because Yamaha used this motor in so many things and still uses it today in the PW50.
I am really sad I had to put it away today because I love it and am glad I bought it.