Ahar!, you can't escape by jumping to an even older thread.

If you've resolved that your problem is the nut, and that it's a Delrin nut, then your task is simple - identify the thread type and pitch and look for a US supplier. Or, consider the replacement of the screw together with a nut it that's easier (unlikely!).

Before that, though, get eyes on the nut and make sure that there is no adjustable component.

Once again, answering this blindly is impossible to give good clear advice. Create your own thread, post pictures.