I converted a 15-91 (solid wheel & potted motor) to a treadle with help from this thread:
Singer Potted Motor converted to a treadle, HC or motor TUTORIAL
I got an after market bobbin winder from Alphasew and per Miriam's suggestion in that thread, I tried needle plate screws to attach it since I had some extras and they worked. I used a nine spoke balance wheel from a singer 128. I had first tried a six spoke balance wheel from a singer 27 but the hub was too deep. My understanding is that balance wheels from 127 & 128 will work, but 27 & 66 six spoked balance wheels won't.
This site has a very helpful explanation of the difference in balance wheels:
http://vssmb.blogspot.com/2011/07/lo...eels.html#more
MJ in TX