I am not sure of exactly what you have in mind. You can try to remove the existing floor tile, float the low spot and retile, but I expect that once you start popping tiles off the floor you are going to damage or run into problems with the walls. If the bathroom is not going to be used once the main bath is complete, can you just clear the drain and regrout? 60 years is not a bad lifespan for a bathroom. It is probably time to do a full renovation. After the main bath is done, you will be used to living around the construction.