The easiest way I have found to get a better idea of what is wrong is to find a way to put compressed air into the spark plug hole of the bad cylinder. You will hear air coming out the carb (bad/stuck intake valve), out the exhaust system (bad/stuck exhaust valve) or out the valve cover with the oil fill cap removed. (broken rings/piston). Most likely the head is going to have to come off regardless of the issue.