If, after severe over-revving, there is virtually no resistance to pulling the starter, and the crankshaft rotates, it sounds very much like a broken connecting rod. You can find out easily by taking out the spark plug and looking through the plug hole and/or sliding the end of a piece of tie-wire down through the plug hole and feeling around, then rotating the crankshaft. If you can't find a piston-head going up and down, you either have a broken rod or a broken piston - and there is no great likelihood of breaking a piston by over-revving, it is usually caused by detonation, pre-ignition, seized rings (no oil) or the old favourite: a foreign body in the cylinder.
You will also be able to see the valves through the plug hole, and verify that they are operating normally when you rotate the crankshaft.
You should be able to get a replacement connecting rod for an eight cubic inch Briggs, but very often when they are broken by over-revving they depart from the crankshaft with enough force to have broken the crankcase, which means a replacement engine is needed.
