Sometimes I have a customer who’s fed up with their rickety old mortise lock but doesn’t want to get rid of it because they are so attractive. Sometimes it isn’t worth it to fix an old mortise lock because either there aren’t replacement parts, there’s too many broken parts to make repairs cost-effective, when locked the door can’t be unlocked from the inside so it’s a fire hazard, etc. In this situation an attractive security solution is to convert the mortise lock to a passage (non-locking) lock and install a deadbolt above the mortise lock.
Mortise locks can often be converted to a passage function lock pretty easily. All that needs to be done is to take out all of the parts that contribute to making the locking functions work: the bolt, the buttons on the side that deactivate the outside of the door, the hub for the thumbturn, etc.
Once all of that stuff is removed you can either replace the cylinder with a flat dummy cylinder (no keyhole) or keep the existing cylinder in there. It won’t do anything since there isn’t a bolt to interact with anymore. You should put the thumbturn back on since it’ll look bad if you don’t.
If your deadbolt matches with a similar finish then your door should now be highly functional, secure and tasteful!