Re: shear pin??
They were so common (at one time) and used for so many years, they should be easy to find. Try a discount marine store, then try a Johnnyrude dealer.
If you get stuck, you could likely make your own out of the correct diameter brass or stainless. My '58 Fat-Fifty used a 5/16" Stainless pin. I don't think it would shear, without tearing up the brass hub on the prop. The 40HP models used a thinner pin(1/4" or 3/16"), and it could have been brass.