Leaking shower & rotted subfloor 53MY

[TXErin]

Hi. My sister & I recently inherited our parents 1973 53MY. Our parents owned it for 25 years. We are cleaning it up and potentially getting it ready for sale. We pulled out the carpet my parents installed when they purchased it, and discovered an issue in the master & mid heads. The shower must have leaked for awhile, and the flooring underneath is rotted. My dad evidently tried to fix the leaking and replaced a portion of the subfloor in the middle head closest to the shower. The master still has the original linoleum and trim, but the floor underneath closest to the shower feels soft. We haven’t pulled it up to take a closer look. I’m sure others have dealt with this, but my searching of the forum didn’t find anything. We’re looking for advice on finding and fixing the leaky shower and the best way to deal with the rotten subfloor. Thanks!