Swapping panels or rollers on a door this age is one thing. Replacing the whole unit on an opening that's out of square, with jamb wood that may not hold new hardware screws the way it used to, is a different project, and it's where we see the most DIY jobs go sideways. If your opening is original to a late-1970s build, we'd rather look at it first than have you buy a door that doesn't fit the frame you've got. That said, we're not going to tell you every repair here needs a full crew - sometimes a cracked panel or a bent track section is a quick fix, and we'll say so instead of selling you a new door you don't need yet.