Hmmm...? Time to earn my keep as it were...lol
OK, are you saying that you can get a drill through the magnet at the moment or have not yet taken it to this level yet? If you have I would be very carefull as to where this magnet wants to stick itself to. It will go straight towards the nearest hunk of metal and that would be your transmission I think, not a good thing IMHO. If you have already drilled through the magnet and it's still in the oil plug I would sticka strongish wire through the hole you have made so you can't loose the magnet like above. If you have lost it already I would be getting the best flashlight you can and shine it up into the transmission and look very carefully for the bits. If you can't find them......I would be taking the left side covers off and all parts to get behind the rotor, starter gear and the DOO idler gear and shaft. That is the biggest hole in which you can fish into that cavity. This where I usually fish for broken DOO parts or spring parts, if not there I go after the oil screen. I never start an engine up without first finding out where all parts are or if I haven't found them after an hour or two of fishing. I would then turn the engine over by hand and go through all gears hoping not to feel a stoppage due to a piece of foriegn material. This where you are now I think or soon will be. Can you post up pics of this mess and also pics of what is left of the plug. Perhaps we can count how many threads are still inside the case and work to figure out how best to get them out without loosing the magnet. Once it attaches to a larger object with a greater pull it will be very hard to get it off without some magical help I'm afraid. I would try and make up a piece of plastic or wood that wouldn't snap off deep inside your engine, preferably plastic, thin is good like a cigarette package thickness....Just something to stop that magnet being able to attach itself to something important. I hope this is as clear as I see it in my head. It's starting to sound muddy....sorry. If you can do this then drilling through the magnet is possible maybe. Even a light tap with a small punch to dislodge the magnet from the plug if all else fails. I think I would try to find a way to tap it back out using a very small punch and a small hammer. You may damage the threads of the case but you can always repair those using a heli coil.
What about using a larger drill and removing enough material on the outwardly side of the magnet so it is retrievable from the outside using a stronger magnet...? Then go in and clean up what is left of the threads. The new larger drill bit shouldn't be that much bigger than the inside diameter of the threads I wouldn't think. It may then just be a retapping to get the threads back to new in the same size.
Hope some part of this is helpfull. Just go slow and think each move through and what conciquences each move brings before doing it.