You need a thin walled socket to get them out as they are in very close proximity to the fins. Before attempting to remove them make sure you blow out the area around them vigorously with compressed air as all sorts of rubbish gets stuck around them, gravel, insect corpses etc. all of which will take the opportunity to drop down the plug hole into the cylinder if they aren't evicted before the plug is removed.
Having been left in for so long they are likely to be seized in pretty tight. When trying to remove them make sure you apply as little sideways force as possible try and keep the extension to the socket as close as coaxial to the plug. If the plug does snap chances are you'll need to take the head off to extract the threads from the head itself.