Japan doesn't have TLAMs, only anti-ship missiles, and those wouldn't be enough to pen the armor of the ship.
It will be sunk by one of the subs most likely there is no ship in the world that can withstand a modern keel breaking torpedo.
Also Japan has DDGs, not CGs.
The JASDF will likely run into munition issues very soon.
That still does not mean that they are fighting in equal terms. WW2 era tech cannot get even close to the modern one.
As for the ammo, I don't see why they would not develop their own? It's not like they cannot do it in a few year to a decade, they already have US personnel and scientists working in Japan today that can assist them in getting their own domesticated development. And giving the entire country isekai situation I don't see why would they not want to assist them?
They can even use their newly acquired allies to get all the cheap resources they have, while their own country specialize in high-tech development. While they slowly bring their allies to our own world level in both tech and social norms.
In short, Japan in this story is like post 1991 US. Only they were smart enough to gain the initiative and shape the world as they see it fit while there is nobody powerful enough to resist them.