The worn stop motion washer is a concern, but you should be able to turn the horizontal top rod by hand with out the hand wheel in place. It's probably in need of more oil and clever rust loosing products. Spray can oils can speed things up, sometimes the basic oil and just letting it seep in for a few days does the trick. It's hard to know which link in the chain of parts is stuck and the trick is to keep adding oil to all joints, hinges, turn knobs, where metal moves against metal. I'm sure you are onto it already. There's a cam like part that moves the feed dogs, it looks a bit like a wrench, inside there is a roller that should turn like a wheel. It often needs a bit of extra attention to free up. The machine will run and stitch, but runs noticeably smoother when it's free.