Hello.
Very respectable and desired car in former Soviet Union states, price may fetch over 100 000 dollars for professionally restored examples easily.
Usually here is found in very bad conditions and then got nut-n-bolt restoration.
There is no survivors at all, but hoards of restored cars. So, better, please don t touch this.
So car is much more valuable as is.
You can find almost anything on this car - so, simply replace worn or lost parts. Hubcaps is not rare at all, although not cheap in mint conditions. Lights, glasses, all kind of moldings - everything is possible to get from former Soviet Union.
But once again - just made sympathetic improvement without nut n bolt restoration.
Also its possible to find some repro parts - as upholstery etc.
There is no direct US ancestors for ZIM. Engine is Dodge slant-6. Body styling very "cadillacsky". Front suspension is upscaled Opel Kapitän. Also you can find lot of stuff from 1930ties Ford (Gorky Car Plant actually was built by Ford to produce Ford licensed cars)
You can find hubcaps and other stuff here.
https://retrodetal.ru/eng/gaz/gaz-12-zim/