I'm willing to be corrected but I'm pretty sure that a repair is not available due to the extremely poisonous chemicals (Cyanide) that are sealed within the sensor bulb and capillary of the ambient gauges.
I believe that the mechanical water temperature gauges can be repaired, but that's not much use to you. I think the only option is to look out for a working one second hand, they do come up from time to time.
Smiths now do an electronic version, but the bezels and faces don't suit our cars
https://www.smiths-instruments.co.uk/classic/auxiliary