A captured Yamato becomes a Soviet victory monument

Would Stalin scrap a captured, partly completed Yamato-class hull after the war?

No: if the hull already had its guns and machinery, the Soviet Union would probably complete it with Soviet equipment and display it as proof of victory over Japan.

The answer treats the unfinished hull as a political asset as well as a construction problem. If the main machinery and guns were already present, the remaining work could be finished with Soviet systems. Completion would advertise that the Soviet state had taken possession of Japan's most prestigious battleship project. The ship's value would therefore lie partly in reversing the memory of Russia's defeat at Tsushima and presenting a visible postwar triumph.