Before you do any of that. Get a compression test done, also whilst the sparkplugs are out check the condition for oil fouling. Also generally check for leaks.
Headgasket would put out plooms of white smoke as coolant would be being burned.
Valve stem seals would only burn oil on a cold start, as the oil collects behind the stem when it cools after a run.
Are you down on power? It sounds more like piston rings to me.