As the others say, its just a case of keep unscrewing the nut until the flywheel comes off. When you start, the nut will loosen and then tighten again as the nut binds on the flyhweel, keep loosening (when you might have to tap the spanner with a mallet) and the nut will draw the whole flywheel off the shaft. Have a couple of bits of iron handy to place accross the magnets as keepers. I am told that this flywheel is self keeping but others say it is not, to be sure I always stick a couple of bits of metal accross the magnets.
Once the flywheel is off it is usually 2 slot headed screws in the centre of the backplate that holds the whole assembly onto the engine. Undo these and the whole magneto should come off the engine.
The next bit is coil/condenser testing to see if it will work !! There are some very good articles in the Stationary Engine magazine that covers the rebuilding of these magnetos however I don't have my copies handy to advise issue numbers, perhaps someone else can advise ?