Neutral safety switch

the starter relay needs to be chassis grounded via its mounting screws and probably is. There are 2 small wires on the relay yellow from the switch gets 12V from switch in start position. The brown wire gets grounded at the neutral safety switch. Check the yellow with a test light. If that proves good Put a jumper wire to ground on the brown wire terminal and try it. Be forwarned that it can start in gear while jumper is attached.