Robert who may live near where Alice entered Wonderland in
Kansas Tennessee, but we aren't sure whether above or below
the entrance, said,
"I see NO reason we couldn't get along
in our Vanagons just fine with 15-20HP."

A few years ago Mechanics Illustrated, I think it was, had a
rather nice appearing build-it-yourself car with kit plans
which used a 3 cylinder Kubota diesel and got something like
130 mpg.   Perhaps there's an opportunity for you in this.
If you want, Martin or I or Alistair can calculate your top
terminal velocity based on matching available Hp. vs.
drag.   From that, we can determine your ideal gearing.
Then all you have to do is find a parked big commercial lawn
mower and   . . . . .
John