Air impact would be the easiest way. It spins it off so fast you dont even have to hold the flywheel still. If you dont have one of those put your wrench on the nut, hold the flywheel with your foot or something, and whack the end of the wrench with a hammer. That will give you enough force to get the nut off.
As for the spark, if your engine has points then just sand those down and you should get spark back.
To get the flywheel off, put the nut back on a little to protect the threads, then hit the end of the shaft while prying being the flywheel. Normally works.
YouTube - fixing the points on a briggs and stratton
Or try what Dave does in this video. You can fast forward to about 2:30 to see what i mean.
Good luck:thumbsup::grind::hack: