While others may disagree, in my opinion you really don't want to do anything that would remove or damage the finish on the wood or the brass.
If areas look dirty you can make up a little water and dish washing soap, going easy on the soap.
Use a clean cotton rag slightly wetted with this solution and gently rub the dirty areas.
If a blackened area doesn't seem to want to clean up with this method, you are not looking at dirt.
Over time, most of the finishes that were used will oxidize and turn black.
While this tends to cover the underlying wood you don't want to remove it. Doing so will expose the underlying wood to the air and will reduce any collector value the gun currently has.
Likewise, don't polish the brass parts. The patina that has developed over the years adds to its charm and its collector value.
After cleaning off the loose dirt you should use a quality paste wax to wax all of the exposed surfaces including the barrel.
