First, let me tell you I know absolutely nothing about up flush toilets. However, it is pretty obvious that the plastic tank must be sealed to be able to use it. I really do not see this as a DIY job. In fact, I have serious doubts that a professional can repair it. My advise to you is to have a professional look at it and go from there. Is it possible to replace the tank? Let me rephrase that. Would it be worth the expense to replace it? Almost anything is possible given enough money to throw into it.