Nice one mate! The mottle is a bit tricky in 1/72nd scale, but it can be done to a reasonable extent. I did mine with a '00' brush, with the paint slightly thinned, and just lightly touched in the mottling 'spots', in a gentle, almost circular motion. Alternatively, you can use the tip of a cotton bud, almost dry. When the mottle is totally dry, go over it VERY lightly, with a broad dry-brush of the base colour (RLM76), just to diffuse the mottle slightly. Once the entire model has had a coat of gloss (for the decals), and then a coat or two of semi-matt clear finish, you should find that the result looks quite good.
BTW, what I've just described was done with Humbrol enamels; you might need to alter it a bit, and/or practice, if using acrylics, as they're not as versatile in use, compared to oil-based paints.
Terry.