Mine went rusty and a couple were a bit chewed up in the philips head (one terminally) so swapped them for stainless from the spares box. Just a regular metric thread, the original has a square nut underneath but a regular hex nut also fits in the slot. Can't quite remember but probably M5 as above, length not too important.
If yours have gone beyond saving and you can't undo it, cut a small slot in the retaining plastic to remove the nut/screw combo - the replacement will still do the job.
Edit: Actually I drilled out the head in order to remove the airbox lid, then was able to get mole grips onto the thread to remove the bad screw. The previous service people (JCT600) had not changed the filter because it was the same one I had put in at my diy interim service (dated with a black marker). My assumption is they chewed up the head, kept quiet and gave up. I had maybe over tightened it, it really only needs enough torque to stop it rattling loose.
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TG