If you can't find a parts and service manual for the model, it can help to play around with the parts close to the stitch width lever and needle positioner. There might be a cotter pin, or bolt fallen out, spring that needs to be put back in place. etc. With just a bit of tinkering and maybe sleeping on it over night, you will find the solution I'm sure. Is it the large rod on top there you move to the left? Look for connected parts going from the stitch width lever to the rod somewhere; a part that should push it in place or something.
