Lawnmower drive-belt problem?

Hello, I have a early 2000s Simplicity 15 horse riding lawnmower. Long story short, the drive-belt came loose at the pulley under the seat. It is on all other pulleys (3 I think) but I can't seem to get it over that pulley under the seat. Do I have to tear the whole back of the mower apart to get it on? Or is there some easy way that I am overlooking? Any tips? thanks so much