For a good and new repro tacho, try this vendor;
http://www.oldtimer-tacho-werkstatt.de/angebote/
The tacho; Isgus Tacho 80 mm
120 kmh Glasskala
Vorkrieg
DKW NZ 350 Behörde
Is a good replacement.
You could try to find an original, but they are expensive and the question remains; How long will this old tacho function? Repairment is very exepensive.
PS. You don't drive with 70 year old tires, so tke a new tacho.
Regards,
Martin