The old Data Marine
instruments are pretty basic and use TTL chips, just one step up from discrete components. I have had a couple apart and effected repair. Never had a bad display though.
It could be so simple as the power supply voltage going to the display is low, that can make an LCD dim. Check all connections and voltages.
Every thing Data Marine used in their production of these instruments was off the shelf components from a wholesale house. The trick now is if they are still available after all this time. What do you have to loose? Open it up and see if you can fine a number on the LCD, then track it down. Try Newark Electronics
they still carry quite a bit of TTL chips. Be a little careful, the LCD is fragile until it's securely in place.