if you want go kart wheels, there is plenty of go-kart parts suppliers on the net for stuff like axles and bearings and such. i think harbor freight even sells parts for them. kinda just hafta piece it together the way you want. im in the same boat (want to build one but havent figured out what exactly i want to build) the trike rear for bicycles usually just have a jackshaft thats chained to a normal rear bicycle rear on one side with a front wheel on the other (with a coaster wheel you still have a brake). the other type thats a little less common has an axle that accepts a keyed hub (basically a fixed gear like a kids tricycle is). both are 1 wheel drive rear ends.