Have a 79 model with same issue here.------Took engine apart.---May cut / grind the crank to save the shaft.-----The splines should be greased about every 5 years or so.
The grease is to lubricate the spring to reduce wear.-----this spring lightly grips the crankshaft.----Should the crank rotate backwards the spring rotates against a contact and stops the ignition.----Oh so simple !