If your tach really is inoperative and you take off the face of the tach (difficult to do without breaking something and there are NO individual parts available), inside you'll find a matchbox sized printed circuit board screwed down, with precision 1% value glass resistors soldered on. After 50 years, a resistor may crack, which opens the circuit. You may need a strong magnifying lens to see the crack. Desolder and it will confirm its condition by falling in two pieces no matter how careful you are. Replace the resistor with its exact 1% value. If you're lucky, thats all thats wrong.
Note- Even if you use precision grade 1% resistors with the exact same color code, this will likely still change the calibration of the meter slightly, so electrical recalibration may be necessary. Some of the other components are electrically 'trimmable'. Then reassemble the tach again without cracking or denting the plating on the chrome face-surround.
So bottom line, unless you are a trained electronics tech, you may be better off to just replace the tach entirely. They have never been cheap. All this is why few 'speedo rebuild' shops actually do more than clean the face of a Veglia tach during their refurbishing (not really a rebuild). All known tachs in DeTomaso cars are the same- Pantera, Mangusta, Longchamp, etc. Not sure about other brands but a broken tach has some value so don't throw it in the trash. Good luck.