I wouldn't worry so much in regards to the fin on The underside, just seal it up so it will not leak. Ironically I welded up a lower unit just two days back. Male strike a rock and punched a hole from the entrance of his unit, dug the chunks https://keylargoyachtrepairservic22221.blogdon.net/considerations-to-know-about-outboard-lower-unit-crack-repair-47915920