Bike might be right, if maybe a bit blunt.
I've never seen a gear-drive Nichols like that. I suspect it's a fairly late-model one, probably from about the time Nichols was winding down production since everyone was moving toward CNC.
As such, there's probably not too many out there, period, and little demand for one to be parted out rather than just sold intact or scrapped outright.
A couple of options:
One, make your own gear. Not easy, but possible.
Two, see if you can bypass the gear drive somehow. I don't know what the full assembly looks like, and you might wind up having to make a bunch of parts, but that, too, might be possible.
Three, how about making a pair of aluminum pulleys and using a belt?
Four, see if you can find an 1150 RPM motor. That'd cut your spindle speed down by a third, and a VFD on top of that might get you low enough to negate the need for the gear reduction.
Or five, as my dad used to suggest, toss the whole lot and take up stamp collecting. You don't need a forklift to move a stamp collection around.
Doc.