The rivets are most likely drive rivets/drive screws. same thing used to hold the metal shield to the bottom of an aluminum intake/or the heat stove to a cast iron exhaust manifold. They have a fast pitch helical thread.
LQQK @ this
As far as the aluminum finish, I would find someone in your area that can tumble those parts, my bud has one in his shop and I do some alternators in his and they look nice when done.
Mike