IMHO if the threads on the post and not the nut are shot, pull the boat and replace the post fitting and nut fitting. If only the nut threads are crossed up, you can get away with dropping the rudder using a halyard
block, disconnecting the quadrant on top of the rudder post and sliding the rudder down so the nut clears the top of the rudder post and QUICKLY replace the seizing nut over the rudder post. Not sure how the Morgan
rudder is configured. If the rudder sits on an extension of the skeg, it will be easier to pull the boat to affect the repair. If the rudder is free hung, you may be able to do it in the water. Don't tighten the nut until you have attached the upper quadrant and repacked the gland. Easy-Peasy but you may take on a bit of water.
Personally, I would pull the boat and replace the whole assembly... then you have no worries. hope this helps... Capt Phil