In rivets, there is not real complexity like finding a machine screw thread pitch, the rivet is only defined by diameter and length, (other than head diameter) information that should be determinable with a fine scaled ruler. If it made overseas, then it may (likely) metric.
I am a little confused by the term "rivet" because if it were a rivet, it would need to be secured through both sides of the material, which looks to be already secured to the shields front. A blind or "Pop" rivet, can be secured from one side only (hence the name "blind rivet") but the other rivets I can see in the picture do not look to be blind rivets. But it is difficult to see what the other fasteners securing the strap mounts are from the pictures.
If it is indeed a rivet then there are a couple of issues.
If it is a not blind rivet, then it cannot be replaced without removing the entire strip it's mounted too from the front of the shield, which look like it would be impossible at this point (or at east without extreme risk of damaging the prop)
If it is a blind rivet, then it may be possible to replace from the back side only, but blind rivets need some extra space behind them for the internal mandrel to go, before the rivet is pulled in tight. This space may no longer be there because the strip is now mounted to the shield front. Blind rivets in soft materials also need a washer on the back side to keep them from just pushing the soft material out of the way when being set.