While I have never ordered one, I imagine getting a OEM motor cycle engine would prolly be a littler harder than a crate auto one as they are such low production comparably. But, if you are into Harleys, Indians, etc, you can get one from a third party builder very easily.
I'm picturing the ELIO, with one of those hot-rod Harley motors poking up through the hood ! If this car makes it into production, you know somebody is going to do this !
Rgds, D.