Just posted on YouTube one-hour ago, late 21 stud motor and the guy is complaining about how everything is different.
Normally I keep a bunch of different "cut" width and length belts in the shop. I never throw old belts away I put the generator/alternator on at its lowest point and wrap the "correct" width belt and cut belt around the pulleys and make a measurement of the belt, then I go and get one that length. Make sure the belt you choose is the correct width, that the belt rides on the beveled sides of the pulley and not the bottom of the pulley's grooves. The parts stores usually have the belts arranged by the width size and length so it's easy to find the one you're looking for, and they will exchange it if you need one a little shorter or longer. I have Gates and Dayco catalogs that list the belts in order of their lengths which helps. I kept all the old parts catalogs from work; they come in handy.
Looks like the same engine we have been talking about.
https://www.youtube.com/watch?v=zNx1dxmuNVg