SolarSys.sc is inside Catalogs0980.pak, which is simply a renamed .zip file.
Some folders that you don't find under addons/ or data/, such as catalogs/, planets/, stars/ etc., are not created by SpaceEngine and usually they are created manually if needed.
You can extract a copy of SolarSys.sc file from the .pak file, make your correction with the text editor, then place the fixed .sc file under data/catalogs/planets/ folder that you'll create manually.