As said, get a new belt. Now that I’ve said, changing this belt is not as easy as most other mower belts. Here’s the order you need to follow.
1. Unhook the drive cable from PP handle.
2. Remove black cover over tranny.
3. Remove belt guide on tranny. Need a #2 square bit. most have a slightly rounded end which I grind flat and square.
4. Remove belt.
5. Tip mower on its side, carb up.
6. Remove blade, and blade adapter.
7. If you cannot remove blade adapter, you must remove belt guide to get belt off.
8. Install new belt and assemble in reverse order.
9. After everything is installed, adjust belt tension.
I’ve done hundreds of these and the time to do the job takes from 30 minutes to over an hour. It all depends on how tight the screws and parts are, plus getting those three screws on the tranny off can be a real PITA. GOOD LUCK.