Or should I attempt to replace it with removable tanks?
You might do better to do something in between. Rebuilding the tank you have leaving it in place could be the easier solution. All my tanks can be removed and hauled out the companion way in about a half day with some help and no destruction of the boat required. In a perfect world that would be what I would want.
You need to fix the problem and consider the idea that you are going to repair the tank properly. You'll get to an OMG moment and decide if a new tank can be inserted easier than fixing the problems in the old tank. Until you can get a good look at the inside of the tank and can inspect the inlet, outlet, and return you won't know really what the problems are. Leaky fittings and a tank clean may be the whole deal or not.
A pro can clean a tank in place for a reasonable fee. I've watched it being done a half dozen times. Being dirty is very fixable. Leaks are bad and can be fixable. Adding a set of accerssible access ports
would be something you really need if you don't have them now.