On a stock 5hp Briggs you should be able to knock all the fins off, as long as you didn't beat on the main body of the flywheel. You could most likely fire it up, open the throttle wide open and walk away. The rod would fry to the crank or piston would melt, or a valve would drop or stick and brake the cam, or the flywheel key would shear, before the flywheel would blow. That's just my opinion though. If I had one to spare, I'd try it myself.