My suggestion is to keep the old one! It a whole lot easier to limit how much water the toilet fills with, than to deal with the restrictions of today's toilets. You can buy any toilet valve, and adjust the fill line to where you want it. If for some reason you still want to limit the fill, all you have to do is add anything of mass that doesn't float to take up tank volume. You could use a few bricks, or a gallon jug filled with water, or just use your imagination. You'll thank yourself in the long run with less clogs that you'll need to call a plumber for.