As I understand the problem, the plug fragment rests TOTALLY INSIDE THE SUMP.
None of the suggestions so far offer any possibility of evacuation and removal, as far as I can visualize.
Either 1) Split cases and remove, or
2) Overbore oil drain hole to a diameter large enough to permit removal of plug fragment. Then, tap larger hole for larger plug (perhaps, "piggy-back" drain plug).
Here's an image of a piggy-back plug; before exclaiming over the projection, hex heads could be ground down to reduce projection, even perhaps the inner plug could be replaced with a hex socket machine screw to eliminate any projection at all from the inner plug:
I'd be inclined to try the latter technique (bore oversize, remove plug fragment, tap hole for larger plug).
If a larger plug is impractical, I'd then have the crankcase built up with weld metal, drill, and re-tap for a standard size plug.
Then, if the plug again broke off in the threads, I'd consider obtaining a left-hand twist drill bit to assist in removal!