Have you tried flattening out the washer/clips with a small hammer, so they will grab better? Of course, if they've cracked, they won't be of much use. As AUS78 said, the mounting studs will unscrew from the ceiling like a screw. Easy to remove with some vice-grips. You could actually replace them with some screws with the same size and length threads as the studs have. They would have to be short. You don't want screws that are too long, since they could dent the outer roof from the inside if they're too long.
