I think it is time for a professional servicing. It was likely so full of dust lint and crud the friction of the mechanics caused the stuff to start to smolder. This likely means carbon scoring and potential heat damage to some critical components. I don't think the zig zag had anything causative to do with it. It just reached a smoldering point.
Sewing machines require fine tolerances and clean surfaces. Cleaning off carbon deposits is not something you can do with even very fine sand paper. You need buffing compound to polish those surfaces again and get the crud off. Unless you are a mechanic with the proper equipment _I_ would suggest you leave this to the professionals.
In the worst case... the innards are fried, warped and scored to the point of disaster. In the best... It was just your motor that overheated and blew smoke into the machine. In any event... there is no doubt some smoke damage that needs attention.