My 1993 honda accord sheered the cam pulley off and so I had the head replaced and it ran for a few minutes then it did it again the mechanic is at a total loss as to why it happened and sugggested we should just replace the motor what would cause the motor to do that

Something seized (e.g. water pump bearing)--the camshaft pulley key was the weak link and gave way. I would figure out what seized and fix it. I would only replace the whole engine as a last resort. Consider towing the car to Honda for repair.