I doubt the oil is coming from the spark plug.
Sometimes the oil can be coming from the left front head stud. That stud has the oil return for the head, and if the copper washer isn't sealing between the nut and the head.....it can ooze oil.
Another leak in that area can occur if the sealant for the head cap fails...this happened on a bike I had. I removed the vent tube and put 5 pounds of air pressure into the crankcase, then sprayed soapy water on the engine....and watched for bubbles.
You need to clean the top of your engine and do the air/soapy water test. If the bubbles come from the washer/nut on the head stud - remove the washer and nut, clean them and install them with sealant under the washer and anti-sieze on the nut. IF the leak comes from the head cap...you will have to remove it, clean it and apply new sealant.
This is what the leak I had looked like with before and during the test:
In this photo you can see where the sealer has oozed out of the joint a bit.
And in this photo you can see the bubbles coming form the joint between the head and head cap...the sealer has failed.