Make or rent a fitting that fits into the sparkplug hole that an air line plugs onto. You can knock the porcelin from an old sparkplug and braze a fitting to it but I think tool supply people have them. Loosen BOTH rockers completely on that cylinder and plug on the air line. CAUTION: the piston will snap to bottom dead center so be clear of the fan. Now you can remove the retainer/keeper/spring with one of those compressors designed to do it on a complete engine. Put a clothes pin on the valve stem to keep it from falling and turn the engine 180 degrees to bring the piston up to where the valve can't fall far. Now look at the keeper groove on the stem, the lock ridge on the keeper, and the taper in the retainer. Work the valve up and down in the guide, it should be drag free. Wiggle from side to side, you shoudn't be able to feel any (or very little) slop. Work your rocker, it too should be drag free. Then to be on the safe side use another spring/retainer/keepers. Good luck

Evan