[frizzen]

That looks like a fun project.

For the drum rotation why not reuse the stock drive gears, stick the pinion on a small slow gearmotor driven off either a cheap esc, or hacked servo board. If you use a standard size servo board the max speed could be set with resistors

I saw a couple other builds where they used a short driveshaft mounted to a nipple coming off the big end to connect motor. I think the drums were strapped in with several bearings mounted as rollers.