It is possible that Klaus is away visiting family or such but he will be able to help you when he checks back in.
So, the first step in the strategy will be to be patient and gather information.
Even though it is important to check the dearing ring for damage, I would first examine the single/multi sleeve at the bottom of the adjustment mechanism to see if it is broken or badly cracked. If it is the original sleeve they tend to crack and become brittle after 45 or so years. It will need to be replaced with a fresh ring to hold the two tiny ball bearings in place as well as cleaning the old lubricant and adding fresh lubricant.
If you force the mechanism or try moving it with the single/multi sleeve broken you will risk damage to the dearing ring.
You may want to order a single/multi ring, some alvania lubricant and a stueurpimpel so that you have them ready when it comes time to service the unit.
Check out the sticky at the top of this forum on how to remove the chassis from the base(plinth). The plinth can probably be fixed with a bit of wood glue.
The lid can be replaced if you want a pristine one by a used one, or you can attempt to repair it if you have the broken sections. If you do a search within this forum there are some good tips on cover repair. Here is one many of such threads.:viewtopic.php?f=37&t=89977&hilit=dual+cover+repair