440 Pistons, How F*cked am I

It looks like just a stock cast piston, it shouldn't be too expensive. IF you can find somebody willing to sell you one, instead of a set.
But, to me, the piece broke off is not significant.
I would sanding roll all the sharp edges smooth, to prevent cracks, and run it. BUT, I would keep an eye on that cylinder, looking at cranking pressure, and for a fouled plug in that hole.