The Singer motor bolt is an odd metric size. M7x1.25 I believe. They are almost impossible to find in the U.S. I've done a lot of internet searching and came up empty.
Sew-Classic does sell them individually as well as in the motor kits.
Opinion = I'd refrain from converting a treadle to electric. They work just fine as treadles. If you want an electric cabinet machine get one already made. Lots of them out there.
Joe