replace an apparently perfectly good tank? If it is aluminum and perfectly dry why replace it? Making a new access plate is trivial and you can make it any thickness you want to make drilling and tapping for fittings easy. Or you can have fittings welded to the new plate.

To replace it you first have to pump it dry. Why not pump it dry and inspect the inside bottom for signs of corrosion. Then decide if it is worth the effort.