How do I replace the drive belt on a 125HP, 38" mastercraft riding mower?

I hope you just left out a decimal point or you have one bad tractor. You need to drop the mower deck all of the way and on most machines remove the blade driver belt, not the one that goes to each blade pulley. Once this guy is gone you should depress the clutch/brake peddal and there should be room to pry off the drive belt from the transmission end.