I don't know where you will find a belt diagram for a snapper unless you go to a dealer.
I have replaced many belts on Snappers. It is really quite cut and dried. What confuses a lot of people is how the belt gets behind the drive plate. A simple trick... Stand the mower up on it's back end. Move the shift lever to neutral. There is a plate at the rear of the machine, which is now almost touching the ground which is activated by the cluth pedal. Put a crescent wrench on this plate so you can "activate the clutch" from the underneath side where you are working. With the belt already removed from the large pulley on the deck, pull the belt around the drive disc. "Actuate" the clutch with the crescent wrench and slide the belt between the dimmple in the drive plate and the rubber driven disc. By using the crescent wrench you are able to move the 2 discs far enough away from each other to slide the belt between them. The install is exactly in reverse order. Make sure when you have it in neutral that you choose a position where the rubber disc is exactly in the center of the dimmple on the drive plate. The belt will then slip right through.
I hope this helps...
Scott Deaver: B&S Authorized Master Service Technician, June 2005
