How to replace door handles in a door with odd mortise locks
We just moved into a 1915 home that had a major remodel in the late 70s. One thing they did was put in three of these wood doors to the outside with lavender glass knobs both inside and outside. I would like to replace the knobs with lever handles, but am having a hard time figuring out the lock mechanism since this one was never fully exposed.
As you can see in the photo, there's a mortise lock, however, the lock mechanism within, directly below the knob, was never exposed/used on either side of the door. Instead, someone put in a separate bolt below the handle (really don't like it below).
So, to replace the knobs (and the spindles are worn, I think), how would I do this? Would I get a whole new mortise lock system with handles, put that in - and also *not* use the lock part? I'm okay with just the separate deadbolt. Or, would I get a passage set (which uses up less space within the door) and patch/fill the area within the door that will be freed up below it?
Also, most exterior/weather-proof knobs are larger than this when it comes to door thickness and scale. Should I just use an 'interior' door knob/lever outside as the prior people did?'
BTW, for what it's worth, the painted side is the exterior, the stained is the interior.