Japan already has a force of 18 quality diesel submarines (not nuclear-powered, though) and have 6 more under construction or planned.
They a 15,000 ton LST that can carry quite a few helos if they wish. I'm sure they could build full-deck aircraft carriers if they felt like it.
They also have Aegis Destroyers.
What would the issue be over? Having a moderate sized, regional based, Japanese navy complete with nuclear power subs might be just what we want.