Singer 760G cam stack drive gear

The "Touch and Sew" series of sewing machine are getting old. The cam stack drive gear is breaking on most 760Gs and people are binning them because they cannot find a readily available replacement. Shame, they don't make 'em like that any more. Fix it with this replacement gear.

There are two versions of the drive gear:
* Early production: A metal worm gear is fixed onto main arm shaft with 2 grub screws. The cam gear is just over 30mm in diameter.
* Later G3 models: Worm gear is cut into