I've got a Sportcity with the Leader 125 aircooled motor in it.
I dunno bout parts in the US, and never ridden the Vespa with the same motor, but I can tell you the drivetrain is TOUGH. Sitting on a steady 55mph with 3/4 throttle cruising along in over 100F temps is easy for them.
It's a small world nowadays, so parts are a simple click away if needed, and servicing is easy with only basic mechanical knowledge and tools if you like to do it yourself.
Check out Modern Vespa for their Wiki info and how-to's.