I have a feeling that the float and magnet are assembled before the two halves of the tank are welded together so reinserted the magnet could be a challenge.
Does any one know where the magnet inserts? Is it in the bottom or side of the float? Any photos of a dismantled expansion tank would be a great help if anyone has a failed tank.
In case anyone is interested the magnet is 2mm in diameter and 20mm long.