What is your ultimate goal? Family boat? Cruiser? River hot rod? If you are concerned with tank replacement and engine swap right now then you should throw river hot rod out the window. These boats take tons of work and $$$$ to make any respectable speed, they are heavy and the pump usually needs to be relocated along with more HP. They make great riding boats and take a pounding, I've tested this many times crossing Elsinore in the afternoon on a weekend! I would keep it if I were you, they are one of the better built hulls from the day. That said, I'm most likely going to have my bubble deck up for sale soon but only to fund my new hallett project.
Do the tank swap, there are a bunch of tanks available right now that should be a close fit. Hit up Madskillz(Nick) if you need them modified for your hull and a little glass work, done.