If it is the alternator, a replacement is available in the USA. Might not be exactly what you want in the end for your finished truck but should get you home. Starters are also available.
This one should drop right in with no wiring changes,
Alternator - Mercedes Benz Unimog U-300, U-400, U-500 Truck 2000-2003; ABO0447 | DBE (dbelectrical.com) . I can check if you post your VIN (I'll look it up in the EPC). The Mercedes part number should also be stamped on your current alternator. It is cheap enough that you can buy it for just the trip home, plus they offer $27 overnight shipping upgrade. Swap the pulley out for your current one (easiest with an impact wrench). Then a metric open end wrench set and you are set. Hopefully the shop you are at could charge your batteries?
I also have the wiring instructions for the more modern dual fan alternator upgrade (which we are currently running). It takes a little bit more time and some extra wiring but I think worth it in the long run.