You can go to searspartsdirect.com for parts list and manuals for many Kenmore machines. See if this link will take you directly to the listings for the 158.1980:
http://www.searspartsdirect.com/mode...l?q=158%2E1980
The last digit is which version of that model you have. As features were added, the number was increased with new releases. Any of the 1980 model manuals would be very similar so you can click "view manuals" under the 158.19800 model and it should help.
If that doesn't explain what you need, there are a couple of vintage Kenmore groups on yahoo groups and there is a vintage Kenmore sewing machines group on Facebook. They all have manuals in files and if they don't have your model, ask and someone may have one to post.
Those "stitch cogs" are called cams.
Good luck.