Gosh,
That looks like a really nice piece of vintage equipment.
Not in the same class of instruments, but Mark Driedger wrote an Article in Circuitcelllar #314 (2016-09) of a pretty nice and simple to build milli ohm meter, based on an "arduino", an ADS1115, a few resistors and a little display. That meter has a resolution of 10uOhm.
The thing can be built for EUR 20 or so, and it is what modern electronics does, but it still does not have the range of your meter.
As you probably know, those meters often consist of a current (voltage) source, and a measurement circuit. When applying an external voltage you can probably determine which of those is faulty. But always start with checking / repairing the internal power supply voltages.
What is the failure mode of your meter?
If it "does nothing" it probably is in the primary power supply circuit and easy to fix, even without a schematic.