the best thing to do is browse ebay.de and just slowly start picking up terms here and there. kolben is piston. rad or felge is wheel ect ect.
germans dont post "gallery" pics (the thumbnails that show up on a search) as much as americans do so you gotta look even harder.
If they are willing to ship. 90% of the time they are willing to ship to america. only major problem is hardly any of them have paypal. you have to do checks. a check will costs you at least 10-15$ to draft in euros and then another 4-6$ to mail 4-6day air.
A complete engine will cost around 60 euros (like 85$) to have shipped over in a week to two.
As far as the engine thing goes, not there are no engines that will bolt on. There are puch 3speeds (x50) puch 4 speeds (monza, m50, n50) and two different kinds of 6 speeds. the monza 6sl which is a 50cc 6speed and the cobra m80 which is an 80cc speed some times its watercooled too. they are very different bottom ends not just the top end. all of these engines have a different mount set up than the standard za50 and e50. however with a LOT of work you can make them fit. they still have the same kind of sprocket setup and can be lined up but require custom brackets and cutting and welding.
Your other option is a manual 2 speed. there are 3 different kinds of these. the 2speed pedal start, the 2 speed kickstart and the Hero-Puch 2 speed 65cc kickstart with optional electronic start. these will just slap right into a magnum or maxi.