Another solution to the low flush toilets is overflow extension tubes. They're basically just plastic piping made to slide over the outside of the overflow tube to extend the height that the water level can fill in the tank. It's also a better solution than buying a new flush valve. I have attached an example of one from NuFlush.com, which has 13 different sizes of tube to fit all overflow tubes:
https://nuflush.com/product/light-blue-nuflush-toilet-overflow-extension-tube-1-175-in-or-29-845-mm-id/
Since all the single packs are the same price, I would order the closest size up, since it will be glued to the outside of the overflow tube. Just leave a message with your measurement when you checkout, and they will send you extra tubes on either side of the measurement along with glue! There is an extension tube category on the website that I linked above. The extension tubes are able to be trimmed easily with an electric or manual saw of your choosing. We would recommend going no higher than 1/2-inch below the flush handle hole.
With regards to the legality of increasing the water level in low-flush toilets, I believe the law is directly applied to the toilet part manufacturing companies that sell parts in CA, where the 1.28 GPF toilets are being placed in new homes. The homeowner can modify their toilets how they please, and it is not illegal to increase the power of your flush. The manufacturing companies are responsible for entertaining the low flush laws.
