In the past, I have used a small sauce pan, put the engine in the pot, covered it with rubbing alcohol, heated the pot on medium low on my electric stove (don't think I would try it with a gas stove, after the engine is warm, about 125 - 150 deg. push down on the valve push rods to loosen up the gunk in the engine.. you will probably have to work the engine and valves a bit, but the combo of rather low heat and alcohol usually clears all that gunk out.. I have used this method on a stuck YS FZ 140, worked well..
Craig.