Just use the SFPM formula. (Motor RPM x 6" x 3.1416)/12 = SFPM then use the answer to calculate the 3" diameter pulley speed. You'll then need to determine the ratio between the circumference of the 3" dia pulley and the total length of the belt. You can use this to calculate the SFPM of the belt.
BTW, take a look at the Surplus Center for a replacement motor.
BTW, take a look at the Surplus Center for a replacement motor.