It is a standard right handed thread, using the right tool helps (you can get them fairly cheaply on eBay) though in the past I have used a hammer and drift. Dog rough though, and damages the nut.
For locking up the motor, assuming it is still in one piece, you could try wrapping an old length of chain around the gearbox sprocket and use a pair of mole grips or similar to hold the two loose ends together and stop it turning, be careful though, maybe use a rag to stop nasty marks. I have done this, again its not pretty but can work. Once apart, ironically it can be much easier to lock up an engine to tighten and loosen things.
Someone may be along soon to express their horror, I'm just saying what has worked for me when in a bind, particularly when I was in a race paddock and not at home in the garage.
