Normal right hand thread.
An impact gun will usually knock it strait off.
Otherwise remove spark plug, bring the piston 1/2 way up on compression stroke and stuff as much cheap soft rope down the plug hole as you can.
Now turn the engine to compress the rope and use a big wrench with a big hammer to give some sharp blows to the nut / bolt it will usually come off fairly easy.
If you can find some one to play the Oboe or clarinet while you are doing this you are performing "the Indian Rope Trick" just dont stand on a loose carpet at the time in case it starts to fly.