I don't know how she would do, but with ANY mare, its kind of a crap shoot the first time around. Some mares make great mothers, and others are horrible in some way or another, I don't think whether or not they were an orphan makes a difference. The instinct should just naturally be there, nature and evolution taking their course to ensure an animal's survival. In my time with horses, I've only met one mare that the owners will never breed again, because she was just not a good mom, and a lot of problems arose, but most mares I've known, even if a little hesitant in the beginning, ended up figuring it out just fine.
I second that the bigger concerns are her injuries, would she be okay and not exasperate her injuries any more, carrying a baby for 11 months. And of course her actual conformation, and the stallions conformation. Being registered does not mean that he horse is automatically breeding quality, and if you are going to breed, you want to make sure that you are breeding a quality horse. I would look at what you want to do with the baby, be it reining, western pleasure, dressage, jumping etc, and then look at your mare, post pictures on here for others to critique your mare, ask people around your area to look at your mare, and ferret out any possible flaws, places of weakness, and only if she is reasonably conformationally sound, then look for a stallion that is built properly for whatever it is you want to do, that he has strong points where your mare may be weak, or need some improvement, and only then should you really breed. Of course us being horse people, we'd love to see pictures just because too.