There would be no problem mounting a Vee twin in front with a coupling in place of the gearbox sprocket to drive the rear wheels, plenty of bike engine kit kars do this, or do as Triking do with a Guzzi power unit driving the back wheel.
The problem comes when you want to sell to modern drivers who want the luxury of a reverse gear, hence the use of a car gearbox

in place of a nice sequential bike box
