On my step, which of course may not be anything like yours, that switch is a reed switch and the moving bit that attaches to the motor assembly to activate it is a magnet. I did have to reglue my magnet in place not too long ago, for what it's worth.
You can easily test to see if it's a reed switch by holding a magnet near it and seeing if it actuates. If it is, there's nothign particularly important about what magnet you use (provided it's of an appropriate strength--a cheap flexible fridge magnet probably wouldn't work, for instance), so it should not be too hard to figure out something that works with a bit of experimentation.