I'm not aware of any dual flush replacements that replace a 3" flush valve.
A dual flush is a Watersense rated bowl
And the Toto Eco Drake and Ultramax are Watersense rated bowls.
There isn't really a savings by making it a dual flush, and the half flush may not work with the larger water surface of the bowl.
The Aquia dual flush has very little water in the bowl. That's is how that one works with a half flush.
Some of the dual flush kits on the market don't clear the bowl.
In fact I saw someone put a 1.5 gallon tank on an old 5.0 bowl one time, and all it did was stir the water.
Right now you have a Watersense 1.28 gpf toilet that works every time. A dual flush is normally 0.9 and 1.6 gallons. When you average that out, you are pretty close to 1.28 gallons.