to open the cap push the cap down and at the same time use a large scewdrider to push on the catch where its broken.
To remove rivet use a largish drill to remove the rolled over end of the rivet and it will push out.
You can fit the later type locking catch which uses any key(or screwdriver) to open or i believe Yamiya, and others, are remanufacturing the old style catch but it is pricey