I have the same portlights and replaced them all earlier this year. They are actually allen key screws, not rivets and those for the handles have a nyloc nut within the handles so you need to remove the little plastic plug in the handle.
found it was an easy job.
When replacing the handle screws you just tighten them to create the right level of friction. For the hinge screws I found it best to loosely fit the screws and then tighten them evenly, checking the closing regularly so the acrylic is not distorted.
I found the easiest way to remove the plastic plugs in the handles was to drill a very small hole in it and then use a slightly larger self tapping screw to pull them out. Once refitted I could hardly see the holes but you can obtain replacements.