My son started the lawn tractor mower and a minute later it shut of on him. he said it sounded like he hit something. I took the engine cover off and pulled the plug. I can manually turn the flywheel about 1/2 turn before he locks up. I can turn it back the other direction same 1/2 turn and it locks up. I obviously know something broke but just trying to find out if anyone can tell what let go from this description. I checked the oil and it is a little low but not empty. I'm trying to decide if I should look for a used motor or undertake fixing this one. thanks