Honda Dominator NX 650, spark plug key

Hello municipality I’ve had an NX 650 Dominator for a year. EZ 09/2002 with 2600km, goiles device problem, the part does not start any more, first the battery weakened, some start attempts but was too weak, then I charged the battery 1 week, now it spins again properly, but still does not jump on (has stood for several months outdoors). Except once a twitch, was not to get out. When starting it smells like unburnt fuel. I wanted to unscrew the spark plug, but with me in the tool there is no spark plug key and with the nut from the ratchet box I can’t reach it, because the candle is too deep sunk and the nut is too thick Now the question, what fits there for a candle plug on it ?? (16, 18, 21 mm) Thanks already for reading ***Staubi***-> hopes that the girl soon came to life PS had tried the search already, but found nothing suitable………..