Mobi is already fully supported, there should be no need for additional plugins. You have several options at this point:
- Use the search & replace feature in bulk metadata editing to fix the data the way you want it. It is definitely possible to fix what you describe in regex mode.
- If you still have the original files, reimport them. Either read the metadata from file contents, if it is good, or read it from filename via a regex (See preferences->adding books).