Most people who deal with Featherweights recommend that you use the old belt, no matter its condition, as long as the belt will run the machine and wind a bobbin.
The reason for this suggestion is that no one seems to be able to make a sewing machine belt that works well on Featherweights as well as original belts. New replacement belts slow down many machines, especially ones with motors that haven’t had their armature turned recently. Some replacement belts work better than others but your old belt will probably work better than any replacement. If a replacement belt slows down your motor it is not doing the motor any favors.
The photograph above shows a variety of replacement belts that have been purchased from different sellers as well as an original Singer belt. All are 17.5" belts and sold as Featherweight belts. All the replacement belts are wider, thicker and most importantly of all they are far less supple than original Singer belts.
Singer Featherweight belts were Singer item number 194144 as the original Featherweight belt in the photograph shows. All the replacement belts have "194144" stamped on them but none look like or feel like the original belt.
With the huge popularity of Singer Featherweights and with so many of them having old belts you would think that someone could produce a belt as supple as the original with the same width and thickness as the original.