I received 2 old gearboxes today from member (gomatthew) to study changes needed.
I worked up the drawing for an adapter plate (about 4" X 7") to hold the sprocket in place.
If the gearbox ever goes bad, you will only need to loosen a set screw on a coupling that connects the sprocket shaft to the gearbox shaft.
Then slip the gearbox shaft out of the coupling. No need to loosen cables, chains etc.
A gearbox swap out should take less than 10-20 minutes. As long as you have removed the wood panel covering the motor / gearbox.
Since the gearboxes are supplied with sprockets, you would also need to remove the sprocket on the new gearbox.