Miriam,
The belt guard / bobbin winder from a clone will also fit.
The belt guard / bobbin winder mounting screws from a clone will fit the Singer 15 head.
I've successfully converted a 15-91 to a hang on motor version using all clone parts. It works great.
Sew Classic has the belt guard / bobbin winder and the spoked hand wheel. I don't know about the screws.
Query; why doesn't the Singer motor mount screws work? I forget at this point, it's been a long while since I did the switcheroo.
Joe