If you can stall it or at least bog it down until it runs out of whatever is causing the runaway, chances are you can save the engine. Years ago I was was discing a field while a friend was plowing the next field over with our farm's open station JD 4320 and, if I remember right, a six bottom plow. After a while I noticed his tractor was really moving across the field, blowing black smoke, soil flying behind the plow, and with him standing up. I thought to myself that fool was really working the tractor too hard but, little did I know at the time, the tractor's engine was running away. It was running on its own motor oil because of a blown turbo oil seal and he was doing his best to get it stopped. My quick thinking friend had put the plow all the way in the ground and was standing on the brakes with all his might trying to stall the tractor when he couldn't get it shut down normally. It finally stopped when it ran out of motor oil. Because the RPM's had stayed down during the runaway, we ended up just rebuilding the turbo because there seemed to be no other damage. We continued to use the tractor for another 5 years until we replaced it with a newer tractor.