There are quite a few reports of people using a vacuum cleaner's attachment to suck the dust cap back to shape. Others have made a paper tube the same diameter as the cap and sucked the cap out themselves. I'd be a little afraid of a vacuum cleaner and would probably go the manual route. As for the cracks, there are a number of rubber cement products on the market for repairing speakers. I'd use one of those to patch up the cracks.