Is it possible to replace the lock on this door that has been painted on?
How can I remove a Mortise Lock and deadbolt from a door?

The paint's no problem. Cut around the door knob escutcheon with a sharp utility knife, so it doesn't rip chunks of paint off the door. Do this anywhere there's paint-glue holding in metal parts. Scrape the paint out of the groves in the tips of the flat head screws using a screwdriver or the knife if you have to.

How do I replace mortise locks with bored cylindrical locks?

That post says don't. Replace it with another mortise lock. Other posts mention having a hard time finding a match for their hardware, you might want to see if you can get yours working again.