Re: 1957 johnson
If it's a 57, it's a 35hp, if I'm not mistaken. The 30 was only made in '56. But, who knows? You'll have to do some more investigation. Look for a round plug on the engine block. It may have the model number of your engine on it. Be aware that a lot of outboards from those days have a different powerhead on them than they did originally, so, you may have a different powerhead than the year of the rest of it.
Take some photos under the cowling and show us what you find.