Sorry, I didn't remember that. I noticed two types of N/O Kwikee switches in this search
, if you can find one that fits and is N/O you will be able to get a better price with one from this search
(had to cast a wide net as I couldn't get e-Bay to list only round switches that were cheap).
These switches are held in with a couple of flutes that catch the door frame, after removing the two screws and metal bracket you should be able to persuade it to come out by working a flat-bladed screwdriver around the lip.
Note that sometimes the security people call N/O magnetic reed switches normally closed because the switch is closed with the magnet in proximity which is it's normal operation. Happens to be what you want but it threw me for a bit until I understood their point of view.