Daihatsu (Toyota Group) made (still makes?) exquisite 650cc twins (sweet, my wife had one) and 550cc triples (the miniaturized version of the 1000cc triple) all counterbalanced and water cooled, with 5 speed transmisison and which would deliver in excess of 60mpg on a steel body.
This baby of a car,. in the 70hp Turbo version would squeal the tires in 3rd gear, and zip in city traffic faster than most Police cars.
On the other hand, a motorcycle engine would be a very poor choice because motorcycle engines are designed to propel a 450lb. bike+ 2 passengers, not a "full-size" city car with all the expected gadgets.
It's a matter of available torque, power band etc.
My personal observation is that Elio wasted a very good opportunity and omitted to make the body as wide as the front track.
I understand that - maybe - Elio are forced to have external fenders in order to qualify as "three-wheeled motorcycle", perhaps if they had an "automobile-like" body it would qualify as a three-wheeled car - if you even have such a thing in your highway code.
But, IMHO, there is nothing against having external fenders AND a wides body for a two-up front seat.
A bit like many racing cars, where the fat radiators sit behind the front wheels.