Had the same issue in my 18.6, used a very sharp exacto knife and worked at it slowly, seams like the sealer "ran" in the lip and became solid. I had a small pin hole, welded them up and put tank back in. Did find the exact fit from Moeller at West Marine when they were in Vic. Expensive, availability was unknown. Welding worked, your pics look "scary" my boats an 07 so looked good when I opened the floor up. Good luck.
HM