I wouldn't worry so much about the fin on the bottom, just seal it up so it won't leak. Ironically I welded up a lower unit just two days ago. Dude strike a rock and punched a hole in the entrance of his unit, dug the chunks and particles out https://boat-upholstery-key-largo87654.bloggin-ads.com/55062561/5-simple-statements-about-yamaha-lower-unit-rebuild-explained