Pros and Cons of sealing a flat roof with liquid rubber product
Adding a sealer to the surface does not sound like a permanent repair - may last a few seasons...

Covering the whole roof again does not sound like a reliable solution. Will this treatment do the 70years the original surface did?

I would look at opening that joint by removing the rivets / screws whatever the fixing is, then re-doing that seal and refixing the metal back down securely.