Sometimes it’s the simple things. If the valves aren’t happy, a dribble of marvel mystery oil or automatic transmission fluid straight onto the valve stem (needle spout bottle works nice), and leave it overnight. It should soften any carbon on the valve and seat, slick up the stem, and leave the valve guides slippery and happy happy. It might blow a bit smoky for a minute, but you should have compression enough to rope start it. I’m mindful of a photo of an early Indianapolis racer with exposed valve train, and the driver is half out of the car oiling a sticky valve on the fly.