I have a Ferris lawnmower with a 20 HP B&S Vanguard engine, Model 351777 Type 1044. The lawnwomer quit while I was mowing and wouldn't start. While trying to start the starter engages the flywheel and the flywheel spins rapidly, but no engine movement. Would the flywheel still spin this easily if the flywheel key was sheared or did the engine possibly sieze? I can't check compression if the flywheel key is sheared, can I? Oil level is good and was just changed (like it gets every year). Lawnmower and most parts are original and 25 years old.