when I built mine I used 18" browning cast iron B belt, (I did buy them from
surplus center),
but if I was rebuilding or building another, it I think I would consider using two donut spare tires, (by adjusting the air pressure one could control the tension,
I did use a trailer hubs for bearings, I on mine used 5 hole but would use 4 hole if doing again, I bolted the hub to the frame and shimmed to get ti true, attaching the spindle to the pulley,
and just hung the B belt on the pulley, it runs loose, on the pulley, for the tire, for the band, I found a belt that was just able to slip on the pulley with out fighting it,
I have an electric start engine on it, and if doing it again, I would use an electric clutch off or like a lawn tractor uses, instead of a manual clutch, (one could have a number of safety switches on the saw, and much less engineering to make the clutch work and keep belts on,
My Two cents,