You need to fit up the gearbox and as you slowly pull it up with the bolts, get someone to turn the motor over from the front to allow the ring gear teeth to kind of grind or "machine" their way into the soft bellhousing metal. There is not a lot of metal to be removed.
Another strategy is to grind the offending metal off but be very careful with a grinder as the magnesium dust in the case can ignite under sparks and cause quite a bit of grief! You cant extinguish a magnesium fire so be very, very careful.
A die grinder is probably the best strategy here. less likelyhood of sparking if you touch something steel in there.
You can buy a bronze bush from VW specialists to push into the bellhousing for the starter. The 12V starter has a smaller shaft than the 6V job so its a simple matter.
Good luck
L8tr
E
Im not a complete idiot, quite a few parts are missing....