Replacing shower valve assembly

Have older home (1940's) with 2 handle shower faucets. Both valves have been leaking for sometime. I have been unable to find the correct replacements so I have been repairing them but they eventually leak again. I bought a replacement valve kit which includes everything I would need to completely retrofit the existing setup, but looking at the setup from the closet side inside the wall their is no valve, only 3 lines that lead to each of the shower valves and the shower head. My question is can buy a kit to replace the entire setup from the front side, inside the shower? I think I need to replace the entire valve assembly including the seat but not sure if this is possible.