A year ago I researched making something like this. With all the different suppliers of parts, it would virtually snap together. There has been a boom in ATV's, and that is what I would use for the front steering, independent suspension, and disc brakes. I would buy a small motor cycle for the street registration and the rear swingarm.
The Honda Rebel 250 has a longer than average swingarm (its "cruiser style") that would allow an ETEK/Agni to mount right onto the swingarm. Both the front half and back half can be found cheap with a fried engine. The only truly custom part is the plastic body, and the faired recumbents pages show several methods for making a body this size by various methods in your garage.
If I was buying a scooter I'd get a 150cc, but to carry two people in traffic, I'd get a 250cc