I believe Jerry is referring to
these.Sometimes it's more about the technique than the tool and in this case you can try something similar. Cut a nut in half with a thin zip disc, assemble the halves near the splined part on the good threads, clamp halves together with a hose clamp, and back it off over the damaged portion. Put a good sized chamfer on the end first to remove galled or smeared material.
Starting a die with no reference to the existing thread rarely follows the pitch as there is only one orientation point out of 360 degrees. This is why the careful filing is done so as to provide a "track" for the die to follow and if not the threads end up "washed out" or "double cut". From the picture my guess is that the thread is 1/2"-13 UNC having a pitch of .076". If so, an M12 thread is only 1/32" smaller on diameter meaning some of the existing threads will be left behind if converting. For this reason I would suggest using an M12 X 1.75 (coarse) die having a pitch of .068" which is much closer than M12 X 1.25 which has a pitch of .049" and would result in reduced thread profile remaining.
