From the ignition switch, follow one of the red wires to a fuse holder. Check that fuse and replace it if necessary.
Check the other red wire and follow it to a 3-pin connector. Make sure that connector is fine, and then keep following the wire to a diode. If everything else is good, then you might try replacing that diode. It might be internal to the motor, though.
I found the parts diagrams for your mower by searching for murray + the model number. They showed the wiring diagram. You can also get replacement parts there.