If you have access to a compressor and an air regulator and can make an adapter to thread into the spark plug hole you can do a little troubleshooting before you disassemble anything. If you put the low cyl. piston at TDC on compression stroke and pressurize the cylinder with enough air pressure to hear leaking, you can listen at the carb air intake, the exhaust pipe exit and also take the oil filler cap off and essentially listen to ring blow by. If a valve is leaking you will likely hear it at either the carb (intake valve), exhaust (exhaust valve) or crankcase (rings). I'm not an expert on failure modes for our engines but I don't recall that sticking valves seems to be a common problem. If a valve stem is warped it needs to be replaced along with the guide. If there is carbon in the clearance between the stem and the guide, it needs to be reamed, finished and checked for clearances to avoid blow by. Either way if that is the problem you will need to remove the head and bad valve. If the valve is burned (more likely with exhaust valves) it needs to be replaced or at least reground and installed with a new or reground seat. It is possible that a valve may become stuck, be freed by removing the rocker arm assembly and whacking the valve stem with a mallet but that is not likely to be a permanent fix. Stuck valves are a sometimes problem for air cooled aircraft engines and since they have two spark plug holes you can fill a cylinder with rope, bring the piston up and compress the valve springs, remove the keepers and drop the valve into the cylinder and ream the guide, replace the valve and carry on. We can't do that so easily. You might see if you can look at the valves with a bore scope and might possibly see if there is a burnt seat if it is in fact valve leakage.