Scags STHM-20KH (3 wheel zero turn) runs but stopped moving

While operating my Scags STHM-20KH mower today it stopped moving either foward or reverse. No linkages from the foot pedal to the engine are broken. I am looking for tips on where to start researching on what is broken and what needs to be repaired. I will likely be calling a scags repair shop but want to educate my self on what will need to be looked at and repaired before doing so in order to make a good decision as to have it repaired or buy a new one. This is a mower that is over 20 years old with 1250 hours on it. Has been operating well with maint. over the years.

Scags STHM-20KH (3 wheel zero turn) runs but stopped moving