SInce the problem seems to be in the cranking side of things, here is how the power flows.
First two principles. The solenoid coil need to have +12 volts on one small terminal ( the terminal with the orange wire). This +12 volts only occurs when the switch is in the "Start" position.
The "ground" side of the solenoid coil has to have zero volts on it for the starter to engage.
So, here is how I would approach things. All tests are done with the key in the "START" position.
1 - verify that +12 volts is present on the small terminal with the orange wire.
2 - Since it is not cranking, you likely have +12 volts on the other small terminal (the terminal with two brown wires. If you measure +12 at the terminal with two brown wires than means that one of the switches or wires is bad.
3 - Measure the voltage at the PTO switch. With the PTO disengaged (make sure the brackets aren't loose) there should be 0 volts on both terminals. If 12 volts in on one and zero on the other, the switch is likely defective. If both terminals measure 12 volts then the PTO switch is likely good.
4 - Measure the voltage at the fwd/rev switch terminals. If both terminals measure +12 then there is a broken wire. If one side measures +12 and the other zero, then the switch is likely bad.