Yes, that is very common. The builder doesn’t care about 20 to 30 years down the road when a new gas tank is needed. I think yours was a gas powered boat so that makes it difficult to cut the tank up in place. I am not an expert on how to cut up a gas tank, diesel is pretty easy since it most likely won’t explode like a gas tank will. Have you done much research on how to cut up a gas tank? The problem is how can you be sure that even if you do everything possible to clean out the gas fumes, how can you be sure you got all of it? I wouldn’t want to make the first cut... If the tank is too big to come out of the hatch and cabin then cutting it up may be your only option, or get a diesel boat. Maybe make a mock-up of the tank out of luan plywood and see if you can find a way to wiggle it in and out. Good luck.