Don't feel bad, I stared at the parts (Harley-Davidson flathead 4-cam set) for an hour before it dawned on me!
Here's one of the cam gears with a substantial center web (look at the thickness of the holes).
A steel disc (very short cylinder) with the OD to match the (cleaned up) web diameter, thickness just below the web depression (to sit flush), and ID to match the (cleaned up) hub OD can't be that difficult? You may have to cut up 2 gears to make the hub and tooth ring a slip fit (unless EDM can do that?).
