Use your throttle to control your speed. First problem solved.
Either you're not tightening your spark plug enough, or your spark plug hole is stripped. It the hole turns out to be stripped, the easiest thing to do is buy a head. Probably better get a head gasket too.
Fixing your wheel is easy. You need to adjust the bearing end play. It would probably be a good idea to grease the hub at the same time. If you have a coaster brake this might help.