Japan has no where near the industrial resources of the USSR, so they'll take significantly longer .
Japan has about half the GDP of the USSR but unlike the USSR
in this POD Japan is not suffering any devastation
(not to mention no spies in the Manhattan Project).
True which saved the Russian an estimated two years but Japan does not need this extra time as Japan nuclear project was already well underway in 1943 and it was making fine progress. Unlike the OTL, this project would not be stopped.
https://en.wikipedia.org/wiki/Japanese_nuclear_weapon_program
I am sure that the Japanese could match the Russians.