It takes the speed of an impact to remove the fan nut but the nut is not on there that tight. If drive had gotten off the v-pulley and ran between it and the fan then fan will probably need replacing too. I just did a Cub Cadet last week where the drive completely destroyed the fan before it quit moving. After nut is remove the nut, retainer, and fan comes off and then drive pulley can be slipped up, the belt install, and then while holding the belt around the pulley its is back in place. Of course the fan retainer and nut needs reinstalling. But first finish install the drive belt so it tension can aid in the tightening of the fan retaining nut.
Just a side note Bert these LT hydros has a very tight to pulley three sided belt retainer. Although you can squeese the belt pass these retainers, it is not good on the belt.
The fan pn is 731-06098 through your MTD parts supplier for around 12.50 USD retail.