Assuming you are looking for an electromagnet, as in the picture above??
Try searching 'parts & accessories', 'crane magnet' in the O scale category of that auction site.
If you're just looking for a permanent magnet to hang from the hook, check a LHS (or web)...lots of small, different shaped rare earth (neodymium) magnets to choose from. CA a loop to the magnet, attach to the crane hook. However, how would it let go of what it attracted?? The "Big Hook 'n Grabber"???
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