if you have ever been to the top of Pikes Peak, you will find an area where water has soaked into the rocks and when frozen actually pulverizes thise rocks into chatt. Water is brutal when it enters objects and then freezes. It literally implodes within.
I have seen entire whole bricks repaired and replaced within the wall section. I agree with tom that if your skills are not up to date, you should seek a professional mason, but if you are, then you could do this. The mortar will be a new cleaner color, but again your wall is painted.