The only thing that comes to mind is BMW. I know some of their transmissions were separate from the motors. The only problem is, I think they were all shaft drive, not chain. I'll bet a chain conversion would be possible though.
I'm actually in a similar predicament with a bike I'm building. I'm using a right angle gearbox from Tandler, and from that a HD type Primary belt drive with a Baker Torquebox RSD transmission. Although right now I'm having a lot of trouble getting info on what belt drive I should use. You could use my method, but there is quite a bit of machine work involved, and quite a few parts vendors with no customer service.
Good luck with the project.