While a key can shear if the shaft taper is not seated properly, it's the taper, not the key that actually holds the flywheel in place. If the taper surfaces of the shaft and flywheel are in good shape, just align it as best you can and torque it to spec. It it's off by a fraction of a degree, it won't make any difference. If, however, the taper is galled from being loose and this is a last resort before scrapping the engine, maybe Loctite will keep it together.