If you haven’t already, try contacting the author of the custom to let them the issue. You can send a private message here in the forums, or you can leave a comment on the custom’s song page in the C3DB.
Anything beyond that will require knowing how to use C3 CON Tools (or perhaps Onyx?) to take apart customs and put them back together.
You are about to leave Rhythm Gaming World and access the external songs DB. While no song or file sold for commercially available videogames is hosted there and only fan generated content is allowed, Rhythm Gaming World has no direct control over the database and no files are hosted here. Alright, we had to made you aware of this, now you can visit the database!