The chap who motorised his for G scale that Dave found the files for, has gone the route of this
View attachment 278034
Driving the rear axle via a pair of printed gears, with rubber band take off to the other two axles.
Personally speaking, the combination of PLA gears, rubber bands and pulleys put me off as I think it'll introduce a whole lot of noise and friction, so I was going to try one of the little 6V flea-drive jobs, driving only one axle and letting the others freewheel
View attachment 278035
It's also worth mentioning that the one I saw in the video was motorised using an LGB motor block