they're usually 12mm something or another, i dont remember the exact name, but its not a normal size, go to an auto parts store and ask around, they might only have ones for car sparkplugs, but the plug should only cost you a few bucks
It depends on if it has over head valves or not, i sagest a Champion CJ7Y, they are for weedeaters/chainsaws, small but make a hot spark, set your plug gap at .35, that will make a good spark under compression.