there are different quality of belts out there. some may barely last a couple mowings, some may last for a really long time.
different belts can hold up to "higher heat" better, and may not really stretch as the belts heat up.
contact power trac, and see what it costs, and then contact a local tractor repair shop / parts store. for pricing info. have model number of tractor and mower deck handy!
a question may come up of how long belt is. if old belt is just cut in one section. see if you can get a tape measure on it, and measure length.
new belt will most likely be a hard fit. so make sure the "adjusting pulley" is not all rusted up and moves freely. some times i can put belt on all pulleys, and then rotate one pulley and belt. and have belt walk / wrap itself onto the last pulley. some times it takes a good size flat head screw driver. to do a little prying of belt. but personally try to avoid prying on belt. i do not do it enough, and when i try to pry on belt, i will more likely put a chip into them. or slice the belt up a little bit.
