well like i said , did u see anything on the plugs? pull the valve covers check inside there first, bent rods? Anything could've happened if he was messing around in there. he could've dropped something down carb. or if he was running it hard he could've over reved it jumped time, cam, dist shaft , threw a valve. Hopefully its something stupid and you'll make him learn how to repair is own toys. good luck and i'd check old oil for metal, might give u a better idea where to start. no metal-top. metal-bottom