The most important thing when you want to repair a mortise lock is to accurately note the description and dimension so that you get the right replacements. You can also take a photograph of it to the store. Also, know the manufacturer of the mortise lock. In most cases, you will find out that the problem is with the cylinder but assess thoroughly to know the extent of the damage. Ensure you buy the correct replacement. Once you have the correct replacement, remove the cylinder and replace with the new one, then rekey your mortise lock.