I took a look at the deff kit online and from what I can see a 1/8 inch rod magnet should work fine.
The main arms are socketed and you won't really be changing the arms much, just the weapons, so you might want to consider just gluing that joint. If you do decide you want to magnetize the shoulder joint. Drilling out and putting a magnet in the body and on the arm socket will work. Just make sure you line them up first the way you want the arms to be positioned. One final thing, not sure how thick the plastic is on the body, but it might be too thin to hold a magnet once the socket has been drilled out. If that is the case, green stuff on the inside to thicken the plastic siding should help.
For the weapons on the main arm, a magnet in the forearm where the ccw sockets with the main arm will work. The sockets for the weapons seem pretty thin, you might need to get another thinner magnet, or another option is a really thin nail. Drill out a hole into the weapon part of the socket. Cut the nail down and place it into the hole with the head part of the nail sticking out. If you line it up properly and sink the magnet in a bit, the weapon should line up flush with everything.
The same above thing could be done for the lower sockets. Magnets into the joint, nail into the arm and weapon sockets. Again, make sure to counter sink the magnet in the joint.
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