I wouldn't stress so much with regard to the fin on the bottom, just seal it up so it does not leak. Ironically I welded up a lower unit just two times ago. Man hit a rock and punched a hole from the entrance of his unit, dug the chunks https://emilioykuem.therainblog.com/30657971/helping-the-others-realize-the-advantages-of-lower-unit-boat-motor-repair-cost