1. In my opinion by releasing a mission to the public, the author has agreed to his mission being modified and ripped a part and re-released.
2. However, always check the readme.txt, which should always be included. If it doesn't hold any specific information on your rights to the mission, go with #1.
3. If the readme doesn't say anything, always remember to still give out credits to the original author. In any case, always credit the original author, it's very important.
4. If the readme specifics forbit you to modify the mission, don't.
5. If there was a readme with the mission when you got it, it would be wise to leave it intact and write your own modify_readme.txt to go along with it, and distribute them with the mission.