Hi,
You can try to clean up the plug of the wire on the solenoid of the starter motor (both plug on the solenoid and on the wire). If the contact is bad (corroded or floating) it give just enough power to move the solenoid plunger until it hitting the shif lever and "click" but not enough to engage the shift lever and activate the starter motor). It solve the same kind of issue several times on friend old timers.
Sorry if I am not using the correct technical wording but English is not my mother language 🤗
