Indeed, just do it. It's likely that the 40 (50?) year-old grease has hardened into wax, but if the bike has simply been neglected and not beat to shreds I bet all you will need to do is clean everything thoroughly (forget "mild" solvents- get a can of carburetor cleaner), fresh grease, and new cotter pins will have it running like new. Even if parts are rusty, I bet the old grease has preserved the insides of the bottom bracket.