Re: what year was my motor made?
Good motors. Its one of my favourites.
You need to check the coils. If they are original they need to be replaced. Keep and eye on the lower unit oil. The seals are probably bad. You need a new impeller. Send me a PM with your email address and I'll email you a copy of the parts manual for a 1957 Johnson, which is mechanically the same as your motor.
The 10 hp was replaced by the 9.5 hp in 1964, which was then replaced by the 9.9 hp in 1974.