I had these "Casement" type window openers on my 1950's era house in Sacramento. I never ran into a double armed one, or one with rivets in the arm. The rivets would seem to be softer than machine screws, but are a much lower profile. If you wanted to go to screws on the arms, you could look at "button head" machine screws. The low profile nuts to go with them would be the big problem to find. I would definitely go with a low profile self locking insert nut, if you decide to replace the rivets.
To get the roller back into the window bottom channel, you may have to unscrew the actuator (crank thingy) from the window frame to be able to move the arm roller into the channel, then re-install the actuator on the window frame.
BTW, when these actuators fail, it is usually the teeth molded/machined at the base of the arm that mesh with a screw drive connected to the actuator handle (or knob) which strip out. Getting this double actuator might be a real problem, so maybe you might want to do your research now while there is no emergency, and buy one for your spare parts stock. Keeping the moving parts lubricated with grease wouldn't hurt, either.