A 50 year old motor.----Some will argue.----Step #1 is a compression test.----Post the actual values here.----Then test charge coil and sensor coils with your ohmmeter.
What does the spline on the end of the driveshaft look like ?----Full of rust?----If so the spline in the crank may be damaged / weak.---A potential failure point.
In my opinion when you acquire a 50 year old motor you need to do a complete evaluation before spending any beer tokens.---Post actual compression values here.----If around 140 PSI ,look for and find a used lower unit.