Retrofitting a 2″ halogen lamp is not possible with LEDs, since a retrofit LED with a G4 base is typically too big to directly replace a 20W halogen lamp.  The best way to replace a halogen puck light is to replace the entire fixture with a 2″ LED can light fixture.  Sunlite manufactures LED fixtures as short as 1″ tall (to replace a standard 20W halogen lamp) and 2″ tall (to replace a standard 60W halogen lamp).