** ONE MORE THING ... DO THIS FIRST! **
Just remembered ... in your version 2.3 there was an 'Upgrade Quiz' (something to that affect) link that showed in the System Admin menu. Run that and convert all your quizzes to the 'new' BEFORE attempting any more backups of those courses.
Sorry about CAPS ... not shouting at ya ... just trying to get your attention!
'spirit of sharing', Ken