Getting Japan to accept them might run into some internal political issues; there's historically been some opposition to nuclear weapons in Japan. See https://en.wikipedia.org/wiki/Three_Non-Nuclear_Principles for example.

That said, if Japan decided it _wanted_ nuclear weapons, it can almost certainly produce them itself very quickly (order of 6-12 months according to most estimates I've seen). They have the raw fissile material due to their longstanding nuclear power plant program, and the general knowhow and infrastructure. https://en.wikipedia.org/wiki/Japanese_nuclear_weapon_progra... has some more data.