what part to purchase to replace

Thanks for reaching out! Given the description of the issue with your operator, it sounds like the worm gear in the operator is stripped. So the handle cannot properly function with the gears inside the operator to open and close the window. 

Your casement window operator reminds me of our 13-1/2" Arm Casement operator, part 39-013. The opposite handed version is part 39-014. Carefully look over the dimensions and compare them to your existing operator to confirm that this is the correct replacement. The compatible crank handle is part 39-114L.

Dimensions for 39-013

Additional information for 39-013

If your crank handle is still in working condition, you may not need to. However, you will need to discern the condition of your handle. It wouldn't hurt to replace it so that you'll know for sure that all of the parts are in working condition when installing the new operators.

I hope this helps!