The part number for the gear is
3692-169S and while I see similar gears on eBay right now (as of October, 2020), they have a 12mm shaft and the part number doesn't have the S. I don't think the 12mm shaft will fit in the Dressmaker. You want one that has a 9.5mm shaft. A quick google search using the part number turned up several sewing shops online that sell them:
https://www.discountsewingmachinepar...3692-169s.html
https://www.sewingpartsguru.com/prod...shaft-vertical
You'll want to scrutinize the photos on these sites and make sure your gear's teeth lean in the same direction, that is important.