Thanks for the lead.Check your local Sears store. Since the Sears parts store closed south of Boston, the local Sears store is now stocking many belts and blades. They are also available on-line at www.sears.com.
The seat might cost more than the tractor, mine does. That's why my seat is held together by 'duct tape'.
One way to judge the age is by the numbers on the motor. If it is a B&S engine, their website has a decoding table. That is, if the motor hasn't been replaced with a newer motor.