The K-4519 tank is the old 3.5 gallon per flush tank.
In the late 80's they went from 5.0 to 3.5 and many plumbers thought performance dropped a bit during that time.
A new 1.28 or 1.6 from Kohler with class five or six would clog less.
You may be able to tune the old one up though.
Water level in the tank is normally about 1/2" below the overflow tube.
Sometimes over time, the trapway collects stuff. We sometimes find that running an auger through may be needed.