You may be able to find toilet repair kits that include an epoxy resin to seal up cracks in porcelain, but because new water is constantly coming in the toilet tank it will just crack further. Every time cold water enters the tank it contracts, then when the water warms up to room temperature, it expands. This constant expansion and contraction will just further the crack. You may not be able to keep up with the leak prevention. I would suggest just buying a new tank from the manufacturer. You can order the right tank directly online if you know the make and model number.