Mounting with a chain and sprocket works fine. Ryland is right about direction of forces though. Maybe an additional bearing and support could be added to take care of that?
I don't have room in the Metro for the motor ABOVE the tranny.
Here is an electric trike done in a way similar to what you are thinking.
The electric motor sits above a Chevy S10 rear axle spun around backwards. Both the motor and axle have sprockets on them, and there is a chain wrapped around connecting the two.
Lots of different ways to connect a motor and transmission together. I went with the Lovejoys because it seemed pretty simple. Plus, that's what the Forkenswift is using, and I am "borrowing" a lot of ideas from that project.
Check out
Shifty's EV car build for another type of coupler.