John, is the worm gear you show above a press fitting? If so could it be pulled and installed on the up grade motor with a little modification as Pete relates above? If not removable is it offered as a separate sale item from Lionel?
I was thinking it may be possible to bore out the worm gear to accommodate the larger shaft of the new motor you mentioned and lock onto the new shaft using a small amount of metal epoxy putty.
Do you think that could work?