I accept no responsibility for anything anybody does except for me. That said: Here is a video showing how to remove a typical antique mortise lock and also how to reinstall it. When you take the cassette out if you intend on disassembling it I suggest you take lots of pictures or even video so that you can put it back together correctly. If the lock says Baldwin or Accurate on it, know that they are very difficult to reassemble if you haven’t done it before.
There may be numbers visible on the cassette case or on a sticker allowing you to order an exact replacement after searching for it on your favorite search engine.
If you are able to determine the broken part there are many parts still commercially available. Kaba Ilco sells split spindles and mortise knobs. They also sell thumbturn discs. Baton Lock USA sells replacement cassettes in numerous configurations along with many different high quality knobs and spindles. Most manufacturers sell mortise cylinders, just make sure that the cam on your mortise lock looks like a Yale Standard cam, if it is different then you have to buy a compatible cam for your lock cylinder.
If you have bigger problems like a cracked cassette case or something contact your local locksmith or mail it to me. Chances are that everybody with the knowledge to make repairs to your old lock is retired or dead but metal fabricators can usually fix these parts. If you want to take a walk on the wild side you can try to get local maker enthusiasts to help you 3d print replacement parts.
Don’t necessarily believe people who say that these can’t be repaired, get a few opinions. Locksmiths are always telling people that it is impossible to fix these locks when in reality they just don’t want to deal with it or don’t know how but with a little time and the will to save money you can and will fix your lock. They are like mechanical puzzles, just think about how the separate parts in your little Rube Goldberg machine works and don’t be overwhelmed.