I agree square threads are a PITA. I did one on an eight inch machine vice a while ago and ended up making my own tool, based on careful measurement of the male thread.
Worth it to me as I picked up the vice for very little and it was in good condition apart from the nut being worn out.
But if you just want a working machine then much easier and cheaper to just replace it with something more widely available, if of course you have the means to do it. If you had to pay someone then the £90 nut might well be cheaper.
The joys of old machinery
