It could be as simple as the start solenoid is burnt out. Some of the newer engines make it tougher to check as the ones with the fuel shut off arm that attached to the stop solenoid you can hold the arm in the run position and crank to get it to start.
I'd start with checking for resistance in the solenoid, work from there. There should be three wires, generally red/black/white. Check between black-red then black-white. One is pull, the other is hold.
