pill the spark plug and pressurize the cylinder. if you hear air blowing out the exaust(when the exasust valve is closed) you have a seating problem with valves. if it pressureizes the crank case then there is a valve guide seal problem. if the breather isnt working correctly it could be forcing oil past the valve gulds and into the cylinder and into the exaust.
pull the valve cover off and see if the oil is able to drain back to the case.
good luck