I do not know this machine at all, but from what you are mentioning here are my thoughts. Possibly the feed dogs are jammed up with fluff, so give them a good brushing out, next does this machine have a darning mode, whereby you do something to stop the feed dogs from moving to enable you to freely move the fabric under the foot? If this is the case then maybe that is why you cannot alter the stitch length as it is set in darning mode.
Have you had the bobbin out and given that area a good clean, checking that there is nothing caught up in the mechanism that might perhaps stop things from moving.
Next how about looking on line for how to adjust the stitch length on that particular machine, may be someone has been tampering with this before giving it to the charity shop, and thus whilst it looks a lovely machine, there is work to be done - so maybe a proper service by a proper mechanic rather than trying to fiddle with this yourself.
Good luck.