We're you able to find the chunks that went AWOL?
Or are they Gandhi?
Doesn't look like great wood or great inletting to begin with so finding a replacement might make more sense.
Harvesting a finger off a wrecked stock might be easier than starting from scratch if you go the repair route.
A 12ga fw would work but you would have to reduce the butt to 16ga proportions with a spokeshave and horseshoe rasp. 12 stocks are dime a dozen compared to 16ga.
All kinds of ways you could approach this.