Honestly, imperfectionist, I'd grab that thing if I were in a position to do so. bama says a rackmount is out due to tire width, and he has as much if not more experience motorizing more different models and types of bikes as anyone around, so I'd take his word for that.
However, it looks to me like there is room in the frame to put in a frame mounted engine, and a jackshaft. With that combo, you could run the power through the pedal chain loop, and there you are. The builder knew what he was doing, the front end is set up for both full Ackerman and zero point steering. Very nice work on someone's part.
Now picture that same front end, but with the triple tree forks forks being shock forks and independently pivoting suspension trianges both side, with a vertically aligned brace triangle centered to support the steering shaft and bell crank. Tie rod/connecting rod ball joint ended to allow flex as the front suspension works. You end up with a front suspension upright tadpole, crank forward design, which would function as a true "leaner", instead of the "bicycling" (only one front wheel on the pavement) most upright tadpoles use for cornering tightly at speed. Add a mountain bike style rear suspension, rather than a fixed rear triangle, and you'd really have a sweet little tadpole softtail chopper.
Oh, yeah - put mechanical disc brakes on the front end of that little beast.
