This is a temporary error/bug that was caused when I added the version number to the mod name. Apparently this breaks saves, so I've reverted the mod upload so this will no longer be an issue for new saves moving forward.
If your save file suddenly says you are missing Zee's Refits, follow the below steps to continue playing your save. Again, this will not be an issue for any new saves moving forward, apologies for learning about this the hard way.

Follow these steps:
- Access your HOI4 mod directory at ...\steam\steamapps\workshop\content\394360
- Open folder 2073318750, which is this mod.
- Then open the file named 'descriptor.mod' with any text editor program.
- Change the line that says 'name="Zee's Balanced Ship Refits V2.1"' and either erase or add the 'V2.1' part as needed, then save the file, and continue your save.