Engine manufacturers, including Briggs and Stratton, use aluminium keys between the crankshaft and flywheel so it will act as a shearable key: when the blade hits something massive, the key shears so that the momentum of the flywheel will not shear the crankshaft instead. The aluminium key is a protective feature: the engine manufacturer is not concerned about the weight of the key, since they can compensate for it when they balance the flywheel. If your crankshaft is undamaged, by far the best way for you to proceed is to get hold of an undamaged flywheel, and put it back exactly the way it was when it was new.
