Crank screw

Thanks for your post! The piece you are searching for is the worm gear from the operator. These aren't sold separately because they often require heavy machinery to install them. 

Since this piece has become stripped from the operator, you will need to replace the entire mechanism. I'm happy to provide a replacement recommendation for it, I'll just need to see a photo of the existing operator.

Be sure to include clear photos of the operator base, the arms, and how it attaches to the bottom of the swinging window sash.

Please let me know if you have any questions!