[toodeep]

Welcome to the forum. Most likely the gear position switch got moisture in it over the years and is shorting out (it's inside the rear cover of the motor). Is yours a foot shift or electric shift model? The light green/red stripe wire coming from the gear position switch is the neutral wire. If you ground that wire the machine will think it's in neutral and start.

 

[toodeep]

There will be no problems running it with the neutral wire grounded. It doesn't rule out the shift drum but most likely it would be the switch (rear cover has to be removed to check either of them).