The old Brazilian motorcycle company Amazonas used an entire VW Beetle drivetrain, engine AND transaxle, for their big cruiser.
The way I understand it they just welded the diff spiders and welded a sprocket to the right side. I have no idea how the shifter worked but as I recall reverse was intact and worked - pretty handy when you have a bike so big that people at the bus stop stand up and dig for change when it goes by.
You could put something like that into a three-wheeler. Air cooled for minimal plumbing, all the engineering is done, MOUNTAINS of go-fast parts if you want to roll that way, and a nice, low installation to keep the CG down around the floor.