I have a Standard 1.6 gal toilet from 1994. Can I replace the inside of the tank to make it a 5 gal flush? Now I have a plastic bucket around the drain in the tank. Inside the bucket is the flapper which lifts up. Can this be replaced with a normal flapper and the plstic bucket removed? Any help will be greatly appreciated. I have 3 of the toilets and non flush very good.