without disassembling the engine, magnets are going to be the way to go. To break the adjustment screw loose you're going to have to get creative. Maybe removing the rocker would be a good idea, and tap it with a hammer on a bench, use a penetrating oil of sorts, maybe even leave it container of motor oil. Lubrication and force are what's going to move it in the end.... Best of luck. Oh, and pics help.