valve sticking

Hi all. New to this site and have a question about how to unstick a valve. I have a 1992 corsica v6 auto trans. I have done a lot of things to this car last year like replace tires, give a tune-up, replace the head gasket, replace muffler and so on. Last year one of the valves started to rattle and I haven't drove it or done anything except to park it and once in a while start it and let it run for a few minutes. Of course it has to be the valve on the rear side of the engine that is the problem. I know to pull the plugs and valve cover and the alternator off to get to it. is there something I can use to clear or dissolve the obstruction, something like Marvels Mystery Oil? I probably will have to remove the intake and exhaust if I have too but if there is any easier way to get that valve right without having to remove everything I would like to find out before doing this job again.
Thanks, Robert