Any piano is possible to bring back to playing condition. It's a matter of how much time, effort and money is one willing to invest and is the piano worth it.
I remember one of the first pianos I ever took in was from a school. I did so for the experience of learning. Every single hammer had been broken off. Most, were broken off near the butts. It looked to us as if someone had just snipped them all off.
My dad helped me figure out the proper hammer line on it. We could see the discoloration verses the cleaner spots where the hammers were striking the strings before broken off. We used those cleaner spots for the start of our hammer line using small strips of masking tape for a straight edge for a beginning line taping them dead center of the clean spots. Obviously, we had to adjust accordingly here and there but, at least we had our beginning line.
I removed all of the hammer butts, drilled out all of the old butt shanks, replaced or re-bushed butt flanges depending on how extensive the damage was due to them breaking hammers off, (with his help of course.) We re-installed the butt's and installed new hammers and shanks in them. We trimmed the shanks accordingly until we found the right height for each section of hammers. The high treble was the trickiest as there is very little to no room for error up there.
The bass strings were completely dead so we replaced them. We also replaced the whites and sharps.
When we were finished with hammer installation, I removed the action, cleaned the strings, re-installed it, regulated it, replaced key punching's etc., leveled keys and whatever else it needed. I even refinished the piano myself. I'll never do THAT AGAIN! That's a trick in and of itself! I didn't realize how many grooves and corners there were in a piano!!! It didn't cost ME much fortunately, as my dad paid for most of the parts and stuff.
When we were finished, that piano sounded and played marvelously! Just like the typical old upright sounded way back when...
I still tune that piano to this day.