Bill:
One source of information that you might check out is the National Archives Microfilm M347: Unfiled Papers and Slips Belonging in Confederate Compiled Service Records. This microfilm contains information pertaining to Confederate soldiers that was not filed in their National Arachives Compiled Service Records. There is no guarantee that anything is there pertaining to John Mangus, but it is one source that I have not thoroughly reviewed in my search for information on men who served in the 42nd Va.