Repair the old brick wall or build new?
If your wall's brick is solid and it stands straight but the mortar is crumbling, it usually doesn't need replacing. Brick pointing, raking out the old mortar and packing in fresh, can keep it going. A wall that leans, cracks through the brick or sits on a footing that's moved is a different story, because patching it just hides the problem. I'll tell you which I see, and if I can't tell until it's opened up, I'll say that too.

