ive repaired them before
cut the shaft off where it will be easy to weld
take a 3/8X24 bolt and cut to length
now carefully and slowly mig or tig weld the shaft onto the cut off portion
taking the neccisary steps to chamfer the edges first
what you cant do is rush the welding as you will damage the lock cylinder and plating
make a short weld then dip the lock portion in water to keep it cool
don't put the shaft in the water
keep going until the shaft is completely welded
then using a file smooth out the weld,then slowly with a file
file the shaft square to fit the lock assembly
clean it up and your done
you cannot run a die over the square shaft to make threads
it will jamb every time
tom