I have fixed my VDO rev counter tacho (engine hour) instrument. The engine hour no longer turned while the rev counter needle still worked fine.
The problem is with a cog that meshes with a worm gear that becomes brittle with age (35 years). Once you have the unit opened up the problem is obvious, as can be seen in attached pictures. I managed to find a supplier of the cog in Germany called Partworks GmbH (https://partworks.de/index.php?cl=details&artnum=2020). They were really quick and helpful by email. They also sent me a repair manual in English (attached), follow the instructions to the letter. Note that getting the rev counter needle off can be tough on the fingers, but perservere, you have to pull hard and it hurts the fingers! If you want any help to fix your rev counter/tacho send me a message.