Didn't Listerclones have a supply or supplier? Bit of a missed trick there, from his point of business view.
Belt will depend on your set-up, like the distance between engine and generator, but a bit of string will give you a good indication. The belt/bearing/ pulley suppliers should be able to furnish you with the standard sizes of belt available. Or google serp belts and pulleys, and go from there.
Me? I reularly use Anglian Bearings in Peterborough for most belts and bearings I need. They also stock, or can source, pulleys etc. You local supplier will be in the yellow pages, I would have thought.
Regards, RAB