A place online does rare earth magnets, forget who. I have two spares so can take a picture and measure them up, or send one over so you can look around.. The earlier McCruise magnets stood out a little bit from the bolt head, later ones seen to be flush. Either way you need some other magnets to stack up and get them out, as they are stuck fast. How one of mine went missing I don't know.
I just measured my two spare magnets. 5.92mm diameter, 6.02mm height. With those dimensions the magnet will sit out a little from the bolt, whereas the latter ones seem more flush. In any case you have to adjust the air gap to suit.