New Idea part # for that belt is 303834. Don't know if that is a valid # yet or not. It also says the effective length of that belt is 101-1/2" long so if you measure the width of the pulley that drives it, you should be able to take those 2 measurements to any equipment dealership or auto parts store and match it up. Or at least get close enough so you can take it back and say "it needs to be a bit longer (or shorter) than this one..." 

Since there is a sliding idler on that drive to tighten the belt, you should be able to vary the length of that belt an inch or so either way.