I'll defer to others on the reverse engineering idea, as I've
not had any recent experience with it.
Another thought...You could make an update inside your
project that would affect all your topics (say, assign a temporary
conditional build tag to all topics), and then close the project.
In Windows, do a search on *.htm at the top level of your project,
and then sort the results by date. The topics with old dates would
be the ones you could delete. After that was done, you could delete
the build tag.
That wouldn't help with graphics, though.
G