Small Engines (Lawn Mowers, etc.): Black sooty spark plug and misfiring, briggs stratton, head gaskets

Question
My engine:
Briggs & Stratton Vanguard 18 HP
Engine Model # 356447 type #0079
It has timing advance key and hi rev kit
Spark plugs NGKs BKR5E
K&N intake filter
Currently being used in a golf cart

One of the cylinders on my engine misfires, causing the
spark plug to come out with black sooty deposits (or is it
the other way around?).  This happens on only one of the
cylinders, the other cylinder runs fine.

I have already:  changed the coil for the back cylinder,
changed the rings on both cylinders, changed the head
gaskets, tried disconnecting the diode harness to see if
there was a change, swapped and changed the spark plugs.  I
don't know what else to do.  I just want to get a
diagnostic.  The mechanics I can handle.

The problem started happening after I overfilled the engine
with oil.   I had to drain the oil, change the intake
filter, changed the spark plugs several times.

Does this sound like a compression, carburetion or igntiion
problem to you?  I am about to do a complete overhaul of
the engine if I can find a solution for this...

Thanks for your time in advance...

Answer
I'd probably try a new breather if you think it's oil.  I'm not sure it would cause only one cylinder to foul though.  You might also have a bad valve seal or head gasket on that cylinder.  I'd probably start with a leak-down test to see if there's something obvious, that might save you a good bit of time and money.