hmmm thats were it gets into the realms of what is regarded as hacking mate, this for sure will get you banned from steam if you go online, and more than likely could contribute to spelling the end of modding in COD if loads of people start doing it.
I think 1C will more than likely permit sound and skins and other surface or exterior replacements. But if you create or give an aircraft guns it never had or increase the damage of the stock weapons, alter fight model/damage model etc, then your are moving outside the boundaries of what will be accepted by 1C, of this i'm sure.