Shower Faucet replacement

We are trying to update our shower. The stems are stripped , so the old handles won't work. (we are currently using vice grips) The plumbing place we took the handles to informed us that our stems are no longer produced. (house built in 1926, not sure when shower added, but definitely before 1965). The current setup has separate controls for the tub and shower, with a two handle setup for the shower. Originally, we were going to get a whole new faucet/valve/rough in kit and replace. This presents problems because our setup has a 6 inch spread, and new rough ins have an 8 inch spread.

We are not plumbers, but are very handy. Can anyone tell me what our best course of action would be?

Can we just replace the valves/stems/faucets and leave the old rough in?
What would be the best way to go about that?

See pictures.

Depending on your local inspector's interpretation, anything other than maintenance to that existing ancient valve may dictate replacing it with one that meets current codes - that would be either a pressure balance or thermostatic anti-scald one. Most of those are single handle fixtures. They do make remodel plates that may be able to cover the old openings. You would need to build a box around the center where that stud is since (unless you wanted the valve offset), it would have to go right where the stud is now. That could get tricky keeping from damaging the tile, but it could be done.

YOu might try a different supply house, you might get lucky finding a stem that would work. Handles, on the other hand, might be harder...generic ones are often a poor fit and could strip out the new stems.