In India, and I've seen it also here in Turkey, they often use additional wheels on a Scooter for handicapped persons; but that won't help you much, as I think it will be a problem to find an importer. Also you can not put the wheelchair on it and the question of lean angle presents itself.
If you want, I could check with the turkish guys who built my and my buddies sidecars- 300$ (with simple freight boat) and it's really solid and the setup was good, too. (see "Winter ride" in my sig line)
The question is, if they make one for the scooter you want, and then the question of freight and customs in the US.
Send me a PM if you want me to check that out