Just go to Fastenal if you have one where you live, or someplace similar that carries a good selection of specialty fasteners, and get a very slightly larger or preferably ask them for an oversize of the same size you have, self tapping screw that's similar in design to what your other screws look like.
So long as you don't use one too large it should be fine to use. Make sure to screw it in the first time out of the case so no tiny metal fragments get on or in your components or motherboard.
PITA to have to take it back out again, I know, but better than it not being fully secured or having to wait on an RMA. Another option is to re-tap the threads with either a larger size or different pitch and then use a bolt or screw or the correct and matching pitch. Again, out of the case.
Make sure to blow off the radiator before reinstalling it to remove any particles. That's the right way to do it, or you could just take two pull ties, snip the head off one of them, stick the other through the hole in the radiator and through the fan, and then take the head from the other one and run it down the ridges until it's tight and snip off the extra.
Or leave it as it is. Heh.