Wow - I don't think you can own them in Japan unless the gun is rendered inoperable (the barrel has a plug welded into it, etc...). The only ones available in Japan are replicas. These must sell at a premium in the US.
Usually here , it depends on condition. Crappy ones around $800 and good ones 2 to 3 thousand. If they have the wooden shoulder stock that attaches to the rear of the grip , up to around $30,000. The replica guns they sell in Japan have really great detail on some. I bought a toy cap pistol of a 1911 45 Colt Auto pistol there and you could swap the parts on it with the real pistol I had.
I think you can get permission to have it re-stamped from the ATF but you would take a risk asking. A good gunsmith might be able to re-stamp it if there is a serial number hidden that has not been removed from it. Some guns have it stamped in 2 or 3 hidden from view places.