I would not worry so much concerning the fin on the bottom, just seal it up so it would not leak. Ironically I welded up a lower unit just two days ago. Male hit a rock and punched a hole during the entrance of his unit, dug the chunks and https://dominickwvrmf.wizzardsblog.com/31657164/5-tips-about-lower-unit-repair-you-can-use-today