I hope you guys can come up with something on the gears. Much easier to replace than a gear box.
Protocraft sells gearboxes, as does NWSL (I believe). As you know, not a simple swap from the Lionel one...
I just reworked a drive-it sucked. Shimming of the drive to frame for proper fit, spacers (custom machined) for axle/drive wheel, and quartering (that really sucked). There were other issues, but those come to my head quickly.
Colorful language was said many times during the project....
NOT impossible-but not for the faint of heart to say the least.