...a whole replacement DeT tach is not hard--they come up all the time at about $200. If you want this one repaired, Bosswrench is correct that a place like Palo Alto speed can do it for about twice that. If you want to move the movement from one working tach (even a Fiat 124 tach you can buy for as low as $20), you still will want some way to move the needle to the right place (calibrate against another tach).
The assumption is that the tach is really bad--honestly, connecting the +12 to the tach input won't kill it (it is isolated by a resistor and cap on the input, so really no current will flow). So if the other tach you've measured the circuit against is another 60/70's Veglia tach, OK, the other may be bad--but any other $15 modern tach from Hong Kong will switch off a low voltage square wave. (the Veglia will not)--Lee