There probably is enough room for a can motor, or a small motor/gearbox combination. If I do try putting together another drive unit, I would use some Lionel jr motor side frames and wheels as the starting point so the engine body wouldn't need modifications. I am still wondering if the Lionel Junction little steam drive unit could be modified to fit the 1681 shell. Below is a picture of a small motor/gearbox that is small enough to fit and could drive a set of wheels directly from the output shaft. It is 3 to 6v, 48 to 96 rpm. That speed should be just about right.
