There are several magnets than can be used and here are a few things to consider:

In a standard Gibson type humbucker the typical magnet used measures .125″ thick, .50″ wide and 2.125″ to 2.5″ in length. The metal spacer under the adjustable bobbin is also .125″ thick. You can find magnets with the same dimensions as the standard humbucker but with different magnetic strength. There are Alnico II, Alnico V, Alnico VII and Ceramics magnets in different grades. Depending on the gauge of wire, number of turns in combination with the stronger or weaker magnet, the tone of pickup will sound brighter with a stronger magnet or softer sounding with a weaker magnet.

Consider the physical dimensions when trying to use another magnet too. If the magnet is thicker you will have to use longer bobbin mounting screws. They are the four round head, phillip or slotted screws that firmly hold the bobbin to the bottom plate. If the magnet is too thick the four screws can become loose and make the hardware in the pickup vibrate and become microphonic. The four screws should firmly hold the bobbins, magnet, and steel spacer in place. As mentioned in earlier articles, if the magnet is reversed magnetically when installed back into the pickup, there can be a chance the pickup will be out of phase when used with another pickup. The four round head, brass, wood screws are a #2 X .5″ and work well when using a .125″ thick magnet. If you use a thicker magnet, you need to use a longer #2 round head-wood screw.