So after studying some Japanese battleships I realized that the IJN seemed to put there ships in service for a long time. And if Japan won ww2 it was likely that if the Yamato survived they would of kept it going for a long long time. The area of this "Modernized Yamato would be the 1990's- 2000.
The difficult part of this is that you have to "invent" Japanese weapons because in modern japan all the navies ships are based on US weapons and ships