The main things that make them throwaways are. (Not in any particular order.) Broken and corroded castings, worn out shaft entry points that allow air leaks, person that doesn't have enough time to spend reconditioning. You can do bushings on leaky shaft openings.
I bought a book and did my Rochester a couple of years ago. The good part is I know what happened, and the boat runs perfectly. The bad part, I spent a lot of time. The really bad part, I got rid of that boat!