Dave, as long as its just the plastic gears on the drive thats sheared, hopefully nothing else should be damaged. The spindle on the drive wasn't broken, so there should be nothing metal thats fallen into g/box internally to really damage anything. You didn't actually mention if the box was noisy or not.
You should get another drive(same colour as the shredded one) and see if that fixes it. Be sure to drain your old g/box oil, and then flush with some fresh oil(leave drain plug off g/box and put at least a litre of oil thru box to flush out any debri. You could use elcheapo no name brand engine oil for this too!)
Top up box with fresh gear oil, go for a drive and see if it works
Good luck!