I suggest you should not increase the drive torque capacity beyond what those two Woodruff keys can transmit, or you will break the crankshaft. The keys are supposed to shear when the machine is overloaded, that is an essential protective measure Briggs and Stratton have provided for their engine.

It does not look badly made to me, and I note that the keys failed, as they were supposed to, without much damage to the blades. Remember that the blades will wear (the machine looks as if it has been used very little or very lightly, in your pictures) and it looks as if when that happens the next step would be to break or grind away those tack-welds so you can replace them with new pieces of spring steel with three small holes in each. You would re-use the original spacers.