I'm just wondering how far that magnet has migrated already... Especially if the bike has been moved. Very expensive "oops!"
The OP can clarify, but . . . my assumption/understanding, the remnant of the oil drain plug shaft rests at the bottom of the crankcase, just inside the oil drain hole . . .
Since the case metal is non-magnetic, the magnet of the plug is of no issue, given the fragment's assumed repose position.
Aligning the fragment with the existing threads, from the outside of the hole, appears impossible. The hole is too small in diameter to remove the plug segment at this time.
Thus, I suggest ENLARGING the hole, drilling it out to a size where the segment can be evacuated downward through the now-larger oil drain hole.
Then, the new, larger-diameter hole can be threaded with an appropriate tap for a larger-diameter drain plug (they're commonly available in incremental sizes, metric and SAE, up to 19 mm (vs. stock 12 mm), perhaps even larger).
I'd think a 19 mm diameter hole, maybe even a smaller one, large enough to evacuate a segment of a 12 mm drain plug.
Yet; ain't MY bike! Tear her down and split the cases, if that's the preferred technique!