Hi new guy.
A gear puller is what you need to do it the easy way. 2 hooked flat bars attached to a crossbar with a threaded bolt. The hooks go under the magnet and the threaded bolt applies tension to the crank to pull the magnet off. A harder way is to use two screwdrivers or prybars on either side of the magnet. Thread the nut on only until it's flush with the end of the crank then give it a whack with a hammer but not like your driving nails, semi gently. It's best to have someone help you with that so one of you holds the screwdrivers under the magnet, prying up as you hold the block and whack the nut, it should pop loose.
But really the gearpuller works better and you will need a puller for other things, too. If there's a used tool place near you I'm betting you find one there or at least a cheaper new set. Don't get one too big or the arms will never fit under the magnet.
One like the puller on the right of the second row is best. The others are too thick to fit in the space you have to work in.