Just too add...
I had an almost identical experience yesterday on my 53A10, coughing, spluttering, running on one cylinder under load, black plugs etc. There was a little bit of oil in the left hand pot, no real smoke though, but I assumed it an exhaust valve not bedding correctly or a valve seat.
I pulled the head off and it all looks pretty fine. Valves sealing perfectly..no sooting on the rocker side..
If only I'd read this yesterday.... thank you for sharing all this. It has an impact around the world and keeps bikes running,
Guy