A glance in the OEP book shows the belt as 5/8 X 29 3/16" OEM # 1902325. The OEP part # is 75-380. Stens shows their part# belt as 265-107 & says it is 5/8 X 29 1/4" long and replaces 97133. So if were me, I would try the OEM belt if you could locate it and then a Stens belt. The Stens belt in my experience is a good belt. I think they are Thermoid belts which actually make many OEM belts. Some of the OEP fall short, though some do fine. Beyond having the proper belt, pulley alignment, bearing or pulley wear will affect belt life as well as an impeller that may be damaged. Another thing to do when installing a new belt is to condition the belt by allowing it to run up without actually putting a load on the equipment for 15 minutes. Hope this gives you some help.

After experimenting w/ different lengths I've found a B-26 V-belt works best and lasts longer. It will not let the cutch totally disengage, but since mine has electric start it's not a problem. I may be wrong, but my theory is the longer belts self destruct due to the clutch idler back bending it too much causing it to overheat. I also found if I hogged wood to it too hard it will make them break.
Just my thoughts,
Mike