2001 Ford Puma 1.6 16v Zetec-S leaking oil inside spark plug hole Cyl one?

2001/2 Ford Puma 1.6 16v Zetec-S leaking oil inside the cylinder No 1, spark plug thread/electrode is covered in clean engine oil. Dry outside before plug removed. The engine is misfiring, lacks power, runs roughly at tickover. I've searched the web, most posts say valve gasket seals and rocker cover seals, but they all look clean from the outside (not removed yet) I think maybe piston ring? seems leak maybe inside/lower down? Red engine warning icon flashes intermitently. John