This same thing happened to me about a year ago. I had to replace the gun motor and the short white arm crank that attached to it. You can buy the parts from Marco. You can try other places and see if it's cheaper.
http://www.marcospecialties.com/pinball-parts/03-8619
http://www.marcospecialties.com/pinball-parts/A-17562
You will need basic tools and pin punch and replacement pin. I forgot the size of the pin but you can measure it with the pin punch.
Good luck
Good luck