The block is cast iron, so it is unlikely that WITH CARE you will bugger up the top surface if you use a steel scraper or (my choice) a razor blade held at a very shallow angle.

Various bolt holes and ports need to be stuffed with paper or other filler material which can be *positively* retrieved once all surfaces are clean. I suggest using kraft paper or pieces of rag in the holes first, followed by Kimwipes (industrial lint-free tissue) on top. Push down with a pencil, then retrieve with a piece of stiff bent wire later. Blow the holes out with compressed air (or use a vacuum cleaner) after retrieving the tissue + rag when you're done.

I wouldn't use anything harder than a wooden paint stirrer or maybe a sharpened hardwood dowel on the piston tops if you choose to attack the carbon deposits.