I guess going through deep mud when the motor is hot sort of bakes the mud on it. I try to clean it off every time i ride but it's starting to become a pain in the azz, so i just leave it there.
Hammer and chisel? I've had success with scotchbrite (green or red) and hot, soapy water. If your case is that extreme, you could always take youe head-pipe off and "wire wheel" or bead blast it. Anywhere you leave mud to build-up, it's going to retain heat...not what you want on an air-cooled engine.