The 3mm Allen bolt (the middle one)that holds the pads in place on my MPH caliper has disappeared.

I know the original MPH was based on the C2, but the design changed for later MPH calipers, like the one I've got.
Finding a replacement bolt is proving difficult, so would I be able to use a C2 split pin instead?
Even assuming it fits as a bodge, would I be in danger of my pads flying out halfway down a hill?

