I don't know what a stitch cog is by your definition but I suspect that you may be referring to the cam stack worm gear, part # 310 007 03. These gears are available, however those produced and sold as after market parts online have ranged from bad to very bad. I would suggest that you find an OEM part (not a reproduction) that are still available from time to time on EBAY etc.
Changing the gear is not as straight forward as it may look. The proper replacement requires: 1. removal of the cam stack without damaging the machine parts, 2. removal and installation of the new gear with the correct alignment of the individual cams, 3. installation of the cam stack and pattern indicator without axial play, 4. correct parabola timing of the main-shaft worm wheel and cam stack gear, 5. synchronization of the zigzag and automatic cam parabolas, 6. pattern indicator timing, 7. checking and adjusting the automatic needle position. SewTechnical Home