If it has 4 wheels, wouldn't you have to register it as a car - regardless of if it had a motorcycle engine in it ?
And I don't see why you couldn't use the Metro engine in a mid engine car .... it would put the exhaust manifold and spark plugs between the engine and the vehicle cab/firewall, but that shouldn't be too much of a hardship.
This sounds like a fun project !
Here's a couple of links to the Ariel Atom - it uses a Honda Civic engine (supercharged ) in a mid-engine configuration. I'm sure its not very fuel efficient but damn it looks fun !!
Wikipedia - Ariel Atom
Edit: Youtube link didn't work for me, here it is for cut and paste:
you tube.com/watch?v=WaWoo82zNUA