Residential & Commercial Security - Locks, Keys & Dead Bolts - Mortise Locks on Pass Through Doors?

We bought a condo that is part of a conversion of a circa 1920's home in Boston. The doors were all once pass through doors. We need to replace the doorknobs/handles on some doors as there are none.

The doorknobs are all alike as follows:

They open via a latch next to a stationary knob that does not turn.

They all once had skelton keys.

Internally they all have long, narrow square shaped lock mechanisms.
We took one of these apart on a broken, non-latching knob and it seems that the internal parts are missing. I am too scared to take a working one apart for fear of breaking it.

Can we fill in the current holes and replace with new doorsets as needed?
I would like to try to keep the current doors as they are nice solid wood doors. Is there a simple process for this? The moderization sets we have looked at all have the solid metal plate that wraps the door, and I don't like this for interior doors.

I forgot to add that there are no dead blots involved here.