[john hall]

Well, sparks are bad--that's one of our mottos at work. So, I would say something has gotten into the flywheel and locked up your engine (creating sparks while rubbing against the flywheel). Or your engine slowly locked up internally and got so hot that the sparks you saw prior to shut down was debris burning away and the flywheel blowing it around. Guess it is also possible something got into the driveshaft or the PTO clutch. Check to see if the driveshaft and PTO clutch are OK, see if you can turn the engine by hand.