most likely they will do as calbear mentioned happened to japan build enough that they can take of the rest them selves. how much of a population drop did happen in japan.
Japan's Civilian ITTL population dropped by 14% from 1940-1947. IOTL the drop was less than 3%. Much of this loss was in the older elements of the population and in the younger, with a disproportionate impact on younger females (12 year old boys can be of use in the Army, girls not so much). This is, of course, a disaster that has follow on effects.
IOTL Japan's population had actually fully recovered and was better than 13% HIGHER in 1955 than in 1940 (83M vs 73M in 1940). ITTL Japan is still not back to 1940 levels in 1960.
Japan was utterly shattered as a modern society. Besides the starvation and destruction of the entire transport network (down to the point that ox carts were a rarity even before the oxen were all eaten), by the end of the war the Allies were conducting large scale air raids on towns with pre-war populations in the 50,000 range because there were simply no larger targets left to burn.
As I mentioned in an earlier post, ITTL
1958 Japan is STILL not quite capable of surviving without U.S. military food aid.
The Japanese would have been grateful if they had only been bombed into the Stone Age.