Sometimes you can be pleasantly surprised when you drop the old one on the counter of a brake n clutch shop. If they are good they identify it quickly and look it up and away you go.
Rebuilding them can be an option but sometimes now labour charges are way over the top but is worth asking about.
Only just came across your post. I have an MB Trac 1000. I'm pretty sure they use almost exactly the same Engine and gearbox as the Unimogs that the army are getting rid of.
Couple of minutes later... I did some research.... and found the following:
The engine is OM352A (code 353.957), the army Unimogs are OM352A (code 353.959).
The gearbox is UG3/40 (code 717.902), the army Unimogs are UG3/40 (code 717.932).
I reckon you should be able to hit up Mog Central in Queenbeyan, Unidan Engineering on the Gold Coast, Independant Components in Adelaide or All Mogs/Quinns Truck Services in Brisbane. Any or all of them will probably have clutches in stock to suit your tractor.
If you'd like to chat with a fellow Unimog/MB Trac enthusiast - hit me up.
Ived had Unimog ex army bits,and the truck clutch was a Fichtel and Sachs single dry plate with a diaphragm pressure plate.......id be very surprised if a tractor didnt have a much heavier duty clutch.............double drive indicates to me me what is termed a two stage tractor clutch,with first stage the PTO ,then drive disengage.....Id also think a sophisticated MB vehicle would have a wet multidisc pack type clutch........in these ,sometimes they slip because the disc pack is too thin,and a farmers fixup is to add one more plain disc ,which allows the other discs to wear down a bit further.