Any attempt to repair a cracked toilet tank is really a temporary measure at best and should only be used for cracks in the lid or superficial cracks in the porcelain. Toilet repair kits use epoxy resin to seal the crack in the porcelain; however, the new water that is constantly flowing into the tank will cause expansion and contraction that will cause it to crack even further. The crack will eventually reach the point that stop-gap measures will no longer work. If the cracks are deep, erratic, or have spidery extensions, the best option is to purchase a new tank. You should be able to get the exact one from the manufacturer provided that you have the make and model of your current tank.