If the fan barely moves, something is wrong with the driveshaft or powerline from the engine and going thru the transaxle. The expensive transaxle parts are probably okay.
I see your tractor should have keys in the shaft. My 300 only has a spring pin and I had that shear. Can you grip the shaft going into the transaxle with a vise grips and hold it from turning. Then try to turn the engine (not with the key start) to see if something has sheared.
Perhaps the internal drive shaft in the trans has broken??