More than likely this is a valve guide seal or possibly the valve guide itself. When doing this make sure the piston is at the top for each cylinder. When you get the seal out see if the valve guide is loose in the head. Try to push the valve guide down into the head or try to rotate it. If it moves your screwed. Slide the valve down into the guide about 1/4" and see if it wiggles excessively in the guide. If there is too much clearance in the guide a seal may work for a little while depending on how bad it is.
Could be a cheep fix but possibly may need a valve job.
Could be a cheep fix but possibly may need a valve job.