Many vintage sewing machines got a belt between the motor and the flywheel of the machine. You will find these belts and some pulleys as replacement parts from many stores. The belts will typically be black V-belts, colored lug belts or round O-rings. This video show the result of a comparative tests with these kinds of belts. The black V-belts got the lowest friction loss in the comparison.
https://youtu.be/pGHbPfaSXl8
Did you know of similar test results elsewhere?