Options to fix an old leaky shower valve
It looks fairly standard to me. I'd contact Eljer &amp; see if Danco, Brasscraft or someone else has put out a replacement that fits. Otherwise, have you tried to rebuild the old valve with O-ring replacements &amp; such? Eljer should be able to direct you to the right kit for the valve.

On the other hand, soldering will be difficult &amp; could cause a fire in the wall if you're not experienced. So, either get a Plumber or get a SharkBite Threaded connector so you can just cut the old valve loose &amp; wrench in the new one. I updated the picture to show the correct valve that has screw shut-offs or stops (the silver screw).