My former friend (who is a Tramp) wanted to do the same thing (so he wouldn't have to walk.)
He bought a 80cc kit off Ebay and picked up a Copper colored Montgomery Ward Bike and put it on.
He blasted about 45 around here for about a month or so &
then the piston welded it'self in the cylinder. It sits behind my house still.
I'LL TELL YA THE SAME THING I TOLD HIM BUY A MOPED.