It is not hard to find the old Bike Bug type engine kits at all. Many are out there under their different names , & some only need a little work to get them going.The best, in my opinion, would be the latter version, with the square type plastic shroud over the engine. That one has a clutch & you don't have to manually lift, by lever, the engine off the drive wheel when stopping. You see them a lot on the BAY. Mike, from the link I sent , can answer about any question, supply parts, rebuild engines, etc.
That would be good to get info on old O & R parts. The biggest problem with these , { all 2 strokes ] is people leaving old gas in them. That & a fuel pump diaphram that dries out. You can find a reproduction diaphram & other O & R info, here;
http://orengines.multiply.com/