Go to a store with a plumbing expert & take a photo with you. My go-to place is Ace Hardware. I actually do most of my shopping at one of the big box stores, but when it comes to something I'm really not sure of, I ask the guys at Ace. They just seem to be so knowledgeable. Don't forget to take the photo...it helps immensely. Good luck!

If it's that old, the valve behind the wall needs replacing anyway. No point in even worrying about a retrofit. Replace it all. And, if the tile install is of the era, that will probably have water damage behind it too and need replacing.

OT: Just want to echo your plug for Ace Hardware. When I need just the right bolt or Dremel attachment, there's always a guy there to help out. I love being able to buy just what I need in the little bags.