The quickest way is gonna be to sandblast it - but if you don't have access to a sandblaster, your next best bet is probably a combination of a wire wheel (on a drill) and a sander (orbital or belt, whatever).
If the door is free of dents, you can quickly mow through the larger areas with a sander, then use the wire wheel to get around bends, dents, etc. and any areas the sander can't get to. Then use some steel wool or sandpaper by hand to get to any areas the drill and sander can't reach.
If you can get it powdercoated cheaply, that's gonna be the most authentic. Otherwise, folks here tend to either use the
hammered or
textured rustoleum spraypaint (on top of a primer, of course).