[EC900]

You done all the easy stuff. Could have a simple hidden vac leak. Tuning to and with a vac leak throws things up for grabs as I’ve found out, incl would be mixture. My vac leak came from under pedal cluster to brake booster, runs back thru tunnel hidden behind engine.
Seems the next step might be an eng. compression/leak down test to get idea on the overall performance. Condition of those old plugs would hold some clue as to type of fouling, as will new ones after a few more miles.
The smoke & oil substance would lead to believe time for a top end. How much smoke and how much oil spews out for how long (substance…water/moisture like bad gas or oil?)? Does smoking go away after few minutes. How’s the exterior leaks, possible head stud break or any noise from valve cover. On a well worn engine, rings and or valve guides has oil leaking into the cylinders is typical when engine sits for awhile.
What’s the mileage or rebuild history should be a clue, but the comp problem/leak test will verify.