The way that I do maps based on existing coastlines is to render some clouds, like usual, put the existing coastline image on top of that and turn down the opacity to 20% or so and apply a red color overlay. Then on a layer between these two I fill a layer with 50% gray and set the blend to hard mix...resulting in the black and white cowspots. On a layer below this I'll do the airbrushing of white and black and push everything into shape. If you don't have hard mix then you'll have to use threshold and undo until you get it right.