I don't know, most of these items will not work, although the Q-car one looks plausible, I think the 3-6 month wait is not the best. I find it hard to believe that the part is so expense, considering that all there is a brass gear box, motor, and an axle. I think I might try a Marx motor. Precision scale should have a free downloadable catalog, if they want business like me.
The Q-car drive is the premier under the floor drive available - it's all hand assembled & tested and the demand for them is such that the wait is 3-6 months.
Perhaps the challenges associated in your current project will put that into perspective for you.
As for PSC, I doubt that their gearbox would work in this application due to size constraints. Some of the NWSL ones might be better suited. I've built power trucks using their gearboxes where it did not matter to me if the motor stuck up into the the car body. There was a good article about how to make those in OST a number of years ago, but I can't find it now.
I'll see if I can get a photo on a drive like that I have on my bench posted.
Another thought would be to adapt Weaver drive components into a drive unit. That would be reasonably straightforward.