Try contacting my friends at EVERSHARP Power Equipment
They've been in business almost 60 years. I'm sure they've seen just
about every tractor ever made and have parts to mowers dating back to
when they opened their shop in the 1950s.
They'll be able to find you a belt and ship it out too you.
There is a form on their webpage. If you don't want to call, fill out
the form and they will call you!
330.723.2428 or 440.582.2100