If you place the lever at zero stitch length, and you got the fabric moving, then best option for the cause of this is that the stitch length lever has been bended. But the lever also consist of the inner part. It may also be bended, but I would be reluctant to start trying to bend it. If the steel is bridle cast iron, then it could break apart, if you try to bend it.
This is a picture of my outer lever:
This is a photo of the inner part:
You are also able to find these parts for sale like here:
https://www.worthpoint.com/worthoped...ngth-253007806