how can I remove my motorbike's starter motor?

I recently had to remove the starter motor from my 1984 BMW K100 motorbike. After removing all the bolts and connections it wouldn't budge so I asked at my local bike school, who also do servicing, and they advised me to "whack it". The starter motor did come out, or rather, most of it did because the 'neck' of the casing snapped off and is still stuck in the bike.
My question is how can I get it out as it's snapped off flush with nothing apparently to get a grip of?