That worm wheel looks like the one I pulled from mine with something like 75% of the teeth worn away. My worm screw was a bit better in that it had maybe a third of the key still in place. The "used" replacements from Monarch were more like 95% and 100%.

I'd give monarch a call and ask them about the leadscrew, they likely have one in the boneyard that you can get for a song.

Look *very* closely at the halfnuts, it sure looks like it may have gotten something embedded and is lapping at the leadscrew. I can't think of anything else that would cause that much wear.

Tom may be right about the nuts being a replacement.
I dug a couple foil type chips out of the nuts. I don't think they have been lapping, but as filthy as this lathe is anything is possible, and maybe the lathe went out of service just after the nuts were replaced.
Harry