Like "Brianator" said make sure those sliders are clean and rust free (looks like you've cleaned them some). Also if you haven't changed the pads you should because the bent one is probably binding. If you can't find pads you should at least try to straighten the bent one. You should be able to find pads. Maybe search for "ATV brake pads".
As for rebuilding, if it were me I would try to find a new caliper that you can bolt on to your existing hanger. (FYI always mount your caliper so the bleeder is at the top so the air will migrate to it). Do you know how to bleed brakes ??? Here is a link that shows some calipers.
www.gopowersports.com
Good luck, let us know how it comes out.