Both of those builders used Just Temperament, not Equal Temperament, although Keith today will make you an instrument whichever way you want. I do not believe that Keith Young ever made and sold an instrument which was not as perfectly fretted as he could possibly make it.
I would suggest to you that there is absolutely nothing wrong with those dulcimers. It's just that they were never made to play with other instruments or be tuned and compared with a hyper-accurate electronic tuner.
IMHO you should not try to "fix" anything, as there is nothing to fix. To "fix" what you see as a problem would require replacing all of the frets, preferably by replacing the entire fretboard.
IMHO if you do not care for the sound of those dulcimers, you should sell them as Just Temperament instruments, or simply not play them with other instruments and enjoy them as they were intended to be played.