Is there a process of repatriation for Japanese battlefield relics from WWII?

My grandfather, rightly, or wrongly, brought home with him an officer's sword and flag from his time in the South Pacific. I have given much thought of late to the question of whether this was ethically the right thing to do, and whether it might be possible to trace whose sword it was and return it to the family. So, the question is twofold: is it possible to do so, and is it something that would be acceptable to do, since I don't know enough about Japanese culture to guess what kind of reaction such a search would garner.