Unless they are really worn to hell, the threads on the tailstock aren't too critical. If memory serves, the cross slide and tailstock lead screws on the Atlas/Craftsman machines were 1/2"-10 Acme
LH thread. Verify that first. If it were me and I didn't trust the used parts off ebay, I would buy a section of rolled thread Acme rod from McMaster car and cut the threads off the old part. Leave at least an inch to turn the threads off of to make a tenon. Cut the Acme to length, drill, then bore or ream so the hole is about .0005" smaller than the tenon. Heat the Acme on the end so it slides over the tenon and shrinks to hold. It can then be cross pinned. There are other methods to get there too. I did exectly what I described to make a new leadscrew from a 24" machine work on my 36" bed. Mine was worn up front by the chuck like they all eventually do and I spliced on a section of 3/4" stock to make it function, with the loss of threading cabability out on the last foot of the bed, which was a good compromise for me.
I'd be surprised if someone isn't recreating a lot of these parts. As was mentioned, Clausing still carrys a lot of the old parts. I would recommend calling. I think this is the correct website
Lathe - Clausing Industrial
There are several Home workshop type forums with a wealth of knowledge. My favorite is
https://bbs.homeshopmachinist.net/ forum.