First, today's codes would not allow that filler valve...the tub spout needs to be above the flood rim of the tub to prevent any possible contamination of the water supply. Then, because it is a shower as well, code requires an anti-scald valve technology. I'd see if I could find or make a waterproof cover plate and mount a new valve, spout, etc. above the rim. It would be an unusual tub that still looked good at 50-60 years, so maybe it's time while remodeling to replace it as well. Code often allows you to repair the existing one, but not replace it. If replaced, it needs to be brought up to current code. Depends on where you live and the inspector.