Toro LX420 tractor mower won't go forward or in reverse.

[mm79]

yeah, you’re right. Still has a ton of slack in the belt. With pedal fully depressed, i can move it at least 2-3 inches each way

 

[bertsmobile1]

First of all an appology,
I got confused
You had it right from the start.
So that leaves a belt that is too long.
It should be a gates 6606BR or equivalent which is a 5L belt 91.125" long ( outside measurement ) .
Stens & Pix both sell a B section ( 5/8" wide belt ) for this application which is the wrong profile.
Go to a real Toro dealer & get a pair of belts as they should always be replaced as a set.
From memory Rotary & Oregon aftermarket belts are the proper 5L profile ( shallower belt ) .