I remember seeing a thread about a (Briggs?) 10HP 1 Cylinder engine being used with a CVT (I think) to power a Metro, apparently it got great mileage but looked hideous.
Correction a Diesel Honda
http://www.teamswift.net/viewtopic.php?t=22429
http://utterpower.com/10hp_chevy.htm
I'm aware this is a swap, not a build, but still is impressive