My 14 years 1.6 gpf sterling toilet tank keep running water to the bowl, I want to replace the tank but California water restriction law doesn't allow purchasing/shipping a new 1.6 gpf tank to California so I need to find a new 1.28 gpf tank instead, is it feasible I can get a 1.28 gpf tank to replace my old 1.6 gpf tank but keep the toilet seat since it is still fine?

Why not just fix the one you have? Sounds like you really only need a new flush valve (flapper).

I agree with HK, that seems like a lot of hassle to solve a $10 dollar 10 minute problem. Its like buying a new car because the tire is flat.

https://www.youtube.com/watch?v=lfbIl2GiZiI