Happened to me too on my '76. I stripped out all the old stuff, made a plexi panel to screw in behind the decorative front bezel, and cut it out to hold new Autometer speedo and tach. Also put in little lamps for hi-beams, parking brake, and back hatch. I included an electrical connector block - I think 11 positions - so I could unplug the whole thing from the wiring harness. Its way better than the old one because the electric speedo can be easily calibrated for different tire sizes and rear end ratios - some are even GPS. So don't panic trying to find a replacement. You can build a better one yourself.
Roger