Well, since you know that one is toast...
I've seen a couple of Geo Metros that were converted to diesel and served their owners pretty well.
There's one on YouTube:
I read an in-depth conversion thread about one that wound up with the head end of a single-cylinder Chinese air-cooled engine poking out of the hood:
http://utterpower.com/10hp_chevy.htm
I'm not sure why, but you may have some luck talking to CrazyJerry (ecomodder user name Changzuki) about that one. I can't recall if he was involved or not. But on a mere 10hp the Metro would eventually struggle all the way up to 50mph, not too bad all in all.
I think a small Kubota or Yanmar three-pot mill would yield adequate power to move the car down the road at a relaxed pace, provide sufficient power to get it up the hills at some reasonable fraction of other traffic's average velocity, and yield BIG dividends at the greasy end of the fueling station.