Before cutting the nut try soaking it with penetrating oil (not WD40), worth trying heat then rapid cooling with WD40 or similar, then after a day or so try an impact driver the shock should free it. DO NOT use fins of cooling fan to lock crank. Remember it has probably never been off since it was originally assembled. Do you have a parts list? If not drop me a pm with your e mail address so I can send one. If you do not already have the date for your machine I can supply this from the machine serial number 56???