You may need to replace the entire filler apparatus. You can buy replacement parts from home improvement stores, hardware stores, and plumbing supply stores.
The newer assemblies do not have a float/arm design, but have a 'donut' float on the inlet pipe. This design is a bit better than the arm/float ball design, IMHO. Thje water level can be adjusted by moving the 'donut' float up ot down on the metal adjusting rod.
It is fairly simple, but the tank has to be drained and all the water dried out. The old filler assembly is installed using a large nut on the bottom outside of the tank. Disconnect the water supply line, remove the nut, and lift out the old filler assembly. There should be a small hose connected to the stand pipe in the tank. You can work it loose from the stand pipe by gently twisting as you pull.
Install the new assembly--with the rubber gaskets in the correct places---the package will have a drawing, rehook the hose to the stand pipe, and install the large nut. Tighten that nut snugly, not tigjtly, as too tight can strip the threads or crack the tank.
Re connect the water line, turn on the water, check for lrasks, and adjust the water level.