You might
-look online in the parts diagrams for your model zero turn and/or the operators manual for the wiring diagram
-disconnect, clean, examine the connector and wire end for corrosion and replace if necessary, then reconnect all the main cables between the battery, frame, engine, starter and starter solenoid.
If those don't help, I would suggest doing some electrical testing. The starter itself is relatively unlikely to fail, other than being run for too long at a time and overheating/melting internally, it's more likely a switch, wire, or connection problem somewhere.