Really the unit wanted is across the top blower Excelsior, not common. Earlier cars had in ineffective across the side blower. They appeared with no blower too.
I wondered what they would go like with a Dolphin Excelsior derived engine in, using the water cooled top end to good effect. A marriage of a certain amount of originality with an ability to use the car. Quads are for four wheeled cars, so not necessarily ideal here. I am assuming that the selection is in need of providing for reverse. My left field option would be a Cossack, Dnieper or Ural unit. One of those has suitable cooling and reverse, is cheap, and has a nice bit of torque without silly speeds.