<< My neighbor has an old yazoo mower and the blade belt broke >>
For many years people have taken broken belts to a parts store to match against
available sizes. A common scheme is to take home several close sizes to be sure
of getting one close enough. If you are a regular store customer, the unneeded
ones can be returned, other store policies may differ. It is also easy to
simply measure the remains to come up with a reasonable approximation to start
with. It's nice to have the parts manual, of course, so keep looking for one.