Hi, mikewood869.
Hold tension on the fan belts and try to turn the engine with the fan or take the cover off the main clutch housing and try to turn the flywheel. Make sure the decompression lever is at start before trying.
Just my 0.02.
1 decompression lever off
2 throttle off
3 put in high gear
4 Lightly bump it with another tractor
Ya have to be careful as you can do a lot of damage if it is stuck and you force things.
Be sure to check the fuel rack to be sure it is not stuck in an open throttle position.
A runaway engine gets pretty exciting very quickly.
Good luck