Your right, finding a part for that shower body might be tough as its so old, I would suggest going to Blackman or Furgerson and asking one of the "old timers" for the part.
You could just replace the entire valve at this point, I would not suggest "make shifting" any type of water supply.
Just think if you "plug it" and then "run out" and then the plug lets go......your finished.
Here are the options I would recommend:
- Search and FIND that part asap
- Replace the shower body yourself ( Only if you are comfortable 0
- Hire A licensed Plumber to repair this.
That's just my opinion
Hope that helps !