They may have been brass rivets.

I have re-riveted the head plate on several of my older sails, with stainless steel rivets.

The mainsail hook is better discussed while you're doing it. There have been lots of posts with techniques, etc.

One simple rule of thumb is pull it up, rotate the sail 35-90 degrees, and try and pull it down. If one way doesn't work, try it the oter direction.