I’d definitely run the engine at least up to operating temp, then hit them with a penetrating lube, let it cool, drive it again, lube, cool, etc a couple times. It’s not always an option though. When possible, it’s the best option though. Sometimes a torch can help, but again not always possible. Working back and forth can help, if you can get them to move at all. My last misadventure had one snap off in the head, then despite a 4+ hour effort of working at it with a torch and lube and easy-out, I broke the easy-out, which then promptly refused to come out and also refused to shatter and then the head had to come off, which resulted in more broken bolts and parts and just really wasn’t a good time, but she runs good now so it was worth it and got me a freshened up top end so it wasn’t fun but it was worth it.
My preference for spray lube is Seafoam Deep Creep or some commercially only available stuff that I happen to have a case or two of yet. Probably going to try some Kroil soon since I’ve finally seen it locally and I’ve heard good things. PB Blaster has never impressed me other than making a decent bee spray for the junkyard. WD-40 is a useful product, but it’s not a penetrating lube.
Also, at least Autozone, and probably other parts stores have spark plug thread tools and stuff for fixing threads.