You need to go to a machine shop with the blade. They will measure the thread with a thread gauge s and run a die over the thread. In most cases however the pommel is damaged rather than the tang. These can be fixed with the correct Heli-coil.
As an aside 75% of those buggered SA scabbard screws were messed up cause most guys do not run the right tap through the holes BEFORE putting in the screw. If you get the proper tap and take the time to do this you can achieve a beauty set.
Oh, the other 24.99%?
They are buggered cause people do not use the correct screw driver!! mad