The original screws are M3x0.5, which are 3mm with a 0.5mm thread pitch, and are 8mm long (measured from the underside of the head). They are stainless steel (I assume 18-8) with a Pozidriv pan head.

The screws in both of the MK2a sleds I looked at have damaged heads, which leads me to believe that a phillips driver was used to install them during manufacture instead of a Pozidriv driver.

McMaster-Carr has a similar screw with a #1 phillips head as item # 92000A118 for $4.66 for a package of 100.

I would also strongly recommend using a low strength (purple) thread locker such as MG Chemicals 8701, Permatex 24024, or Loctite #222.