My wife's Singer sewing machine has stripped a gear. It is the gear at the bottom of the vertical shaft that transfers motion to the bobbin. I have attached a picture of the stripped gear located at the bottom of the machine and a picture of the top of the machine showing the gear at the top of the vertical shaft. I have changed some gears on the machine in the past but I have forgotten how I did it. This operation looks like there is no way to get there. Any help would be appreciated.