Is that a ball bearing? I'm only guessing, but it looks like the top is crimped to the bottom piece around the circumference of the flange. You could very carefully grind off the crimp, leaving enough metal for the rivets to hold onto. That should separate the two halves. Once the rivets are back in, they will hold the two halves together.
However, the top half looks like itself is two pieces. (Picture 2.) I can't quite tell from the picture how they're joined, or if it's even two pieces.
Mind you, I'm only guessing here, never seen anything like this before. If the bearing is good, I might just try to squish as much grease in it as you can, and reinstall.