The "original" belts the FWs came with were "V" belts, not the round rubber band belts. The correct belts can be had from Sew-Classic as well as the LSMG. The motor also has some adjustment built in as well.
The correct belt numbers are: Singer and most aftermarket V Belt; 194144. Alpha Sew amber cogged belt; 1712.
Just remember with a correct type black V Belt, the belt needs to be just tight enough to not slip. Any tighter and that will put a big strain on the motor.
With the amber cogged belts you have to set them tighter, but not really tight.
Joe