how do I repair mp3 files?

Try playing a file in Quicktime Player. If it plays it probably isn't corrupt but having work done on an external drive could easily result in iTunes not recognizing the old location for the files. Was the drive renamed? You could try quitting iTunes, renaming the drive so it has exactly the same name as the old drive, then start iTunes again.

Were just media on the drive, or the whole iTunes folder?

You can try mp3 Scan&Repair and MP3 Trimmer (and neither do AAC), but if the files are truly corrupt then there may be no way to fix them.

If repairing the drive simply meant putting a drive corrupted by, say, a bad power source into a new enclosure then you can try a repair utility like Disk Utility or Diskwarrior ($100) to see if it can repair the file directory and unscramble the pieces. What does Disk Utility say about the drive if you verify it? If these are truly unique files then I wouldn't try repairing it yet because sometimes it can make it worse and if you are absolutely determined to recover the files then it may prove very, very, very expensive to have it done professionally and you don't want to have it made worse by a botched repair job.