I believe it's an M3, with a course thread. It's only 4mm in total length, head included. There's only 2 threads showing, it's cut quite short.

Since it's being threaded in to plastic, you likely have some flexibility for finding something that'll work, but may not be exactly what SpeedPlay included.

In my stash of electronic/computer screws, I see a few potential matches. If you can't find something that'll work in the hardware store, perhaps a local computer store will have a few.

Thanks, great!