The Japanes can build overnight a nuke force the way the Saudis are contemplating it. And their missiles are much more precise. They also have a vast civil defense underground tunnel complex. They can rebuild in no time.
The Japanese have all the components they need. They have a mature space program and can easily build the missiles they need for nukes. They have more than enough fissile material to build plenty of nukes, and probably are just waiting for a go-ahead to assemble already manufactured bomb components.
The one thing they’re short of is manpower, what with an aging population that is a fraction of the size of Chinas. They make up for that with scientific expertise and a robust self-defense capability (they don’t buy antique ships from former USSR states).
My thoughts are they already have a few nukes. They have the brain power and the raw materials.
There is a book written by an Army intelligence officer right after WW2. He claims that the Japanese were closer than the Germans to developing a working nuke. (the reason we nuked them so fast) He says the lab was in Tokyo but moved to Korea out of reach of US bombers towards the end of the war.
They found evidence of a dismantled lab with traces of radiation where they thought it was. The scientist disappeared after the war and the story is they along with all the lab work was taken by the Russians. (reason why the Russian nuke program happened so quickly)
The Japanese confirmed that the scientists disappeared. They have been moot about the nuke development efforts though. The Russians never confirmed nor denied what was in the book.