The NorthWest ShortLine Stanton Drive is available in S-scale for your custom projects, or for remotoring an existing model. NWSL carries 8', 8'6", and 9' wheelbase versions, with 33", 36", and 40" wheels, with a wheel thickness of code 110 (normal S) and code 87 (Proto:64). These are available powered and non-powered. If your engine doesn't need to pull much, you might save some money by only having one truck powered and use the non-powered one for the other. The Stanton drives are DCC-ready.
Jim King provided the following information about how to adjust the wheelbase of the Stanton Drive. Remove the screws holding the PC board. Lift one axle at a time just far enough to clear the gearbox. Then, rotate the off-center bearings, and put back in the gearbox slot. Rotating either set of bearings give you 6" of scale movement, or do both of them for a 1-foot adjustment. While you're there, Jim suggests placing a couple drops of RC plane oil on the gears (he uses Trinity Royal Oil found in the RC plane section at Hobbytown USA), but likely any super-thin oil will work.