Ships like that generally have few basic texture maps which contain all the details needed to cover the hulls.  By applying layers in a either Photoshop or GIMP, you can brush in finer details here and there, but if you are considering a major overhaul, then the project becomes more involved.  This is particularly true if additional maps like displacement/bump, specular, and normal are necessary to enhance the final image.  Replacing sections with a combination of procedural and image textures will help in the long run, and introducing other effects like reflections will make the model a little bit more realistic.