[92snowmachine]

when i was considering doing a similar project for my nephew i was planning on using polini motorcycle parts. they make those mini-moto things but are actually just miniature versions of super bikes not the cheap pieces you get at auto zone. they sell a viscous clutch that you can change the springs to get your desired speed without having to use gears. my brothers polini bike goes 45 mph and only has one gear. the wheels have a cable operated disc brake system on them so you could use. small motors typically spin to high enough rpms that you only need one gear unless you want to go crazy fast.

p.s. 45mph inches off the ground feels like 200mph