Welcome to the Krag world. 
Not sure what you mean by the "front sight ring screw".  There is no ring or screw on the front sight of Krags.  However, many Krags had the barrels cut back to shorter length and 1903 Springfield front sights were installed.  These had a "ring" around the barrel and also a tiny screw.  These screws are available from several sources.
As to cleaning the wood...it all depends on what it looks like now.  The one rule here is not to remove any wood unless you have to.  "Cleaning" is okay.  Removing wood with sandpaper so it looks clean is not. 
The easy answer to your question, with no additional information, is that yes, cleaning is okay for your stock and could possibly make the cartouche show up better.  However, the devil is in the details...it all depends on how you "clean".  A little Murphys Oil Soap, turpentine, or lemon oil on a rag rubbed very briskly into the wood should be okay.  These are pretty gentle ways to clean the wood.  Anything strong than this and you could do more harm than good unless you know what you are doing.
Any possibility of posting photos?

daveboy