Yes you can, states that don't exist anymore have other states that inherit both their treasures, secrets, copyrighted works as well as debts. I understand that Russia is that state in general. The issue is complicated in case of ex Soviet Republics presumably.

But I would guess you also can't just storm into Mosfilm or Lenfilm (movie production studios) and start copying all their movies made during the Soviet times and selling them for profit.

Not sure if this applies to the maps in question, but legal rights (and obligations) are handed down to successor country/countries. International treaties, debts and everything. Even Olympic medals I believe.