My Twin 46 won't start

I have been given a Murray lawn tractor - model 46102X6B. I have been using it for a while, but now it won't start. Feels like it may be a safety switch problem. I have replaced the solenoid last year. There is 12V on the battery side of the solenoid until I turn the switch to start, then no voltages on any of the posts of the solenoid. If I connect the battery directly to the motor side of the solenoid, then the starter engages.
How can I tell which switch is not working (assuming it is a switch problem), and get a new one?
Thank you,
Matt

My Twin 46 won't start