I very much doubt that a lathe with 6tpi Whitworth or Unified thread would have the same register dimensions. Possible, I suppose, if you are looking to sell. Check out the wanted section on the forum.
Bore out and sleeve it, if there is sufficient metal to do so. Then recut with a square thread.
I see second hand Raglan back plates selling for around £60 delivered, on epay. They are easy enough to make – and one would be confident there is sufficient metal to safely cut another spigot.
There are threads, for making spindle nose protectors, on :
**LINK**