I used to calibrate tachs once in a while, but I had access to some nice test equipment where I worked.
First, determine if it will come apart. My Stewart-Warner tach does not as the trim bezel is swaged on. But most tachs do come apart.
Inside there are generally two adjustment potentiometers. One is to zero the tach, and the other is for the tach gain when an ignition signal is present.
To do it yourself:
1/ remove the cover
2/ turn the igntion ON, and determion which pot is for ZEROing by trial and error. Zero the tach. (Don't leave the ignition on too long, as it may damage some of the igntion system.)
3/ start engine with a known-to-be-good test tachometer. Adjust the other pot (GAIN) so that both tachs read the same.
4/ re-install cover
I'm not familiar with the tach you have, so I can only speculate on the lights. By it's age, it must use bulbs, so maybe they're burned out (or the filiment is bad from vibration...or the bulb holders are rusted, or a build-up of corrosion on the tinned center of the bulb, etc.), but there must be an external adjustment to set the warning level.
After the tach calibration, (and checking of the bulbs while the cover is off), see if the external adjustment works.
If you really don't want to try it yourself, you could try these guys. I have no personal experience with them, but you could call them and see what they have to say:
These guys seem to have a good rep, and could probably provide the service despite the fact they specialize in Smith's and Jaeger:
And I found this place recommended on the net, although they also seem to be mainly interested in British instruments:
Mo Ma Manufacturing - instrument repair
13211 Second Street. NW
Albuquerque, NM 87102
Ph (505) 766-6661 - Fax (505) 766-5419
I suspect that even though calibration is easy, any company providing this service won't be cheap. If you arn't comfortale messing with it yourself, maybe ask a friend who's an electronic technician to help you. A few beers might be all it costs!