You can custom-mold another arm if you can find one that would look good. Or - a bit trickier - you can copy the arm that is there and reverse the hand.
I have done this many time with broken figures. The top of the copy will have no features - unless you copy two halves - so plan acordingly; but it will provide a servicable replacemet.
Get some epoxy putty; make a square a little larger than the arm; coat the arm with vaseline, olive oil or some other "release agent"; press the arm into the the putty square; gently remove it and let the putty square harden. Now you have a master mold.
Assumiing you are not into casting, you probably won't have any spare metal to make the arm. You can use other materials, or I just use more of the epoxy putty. Coat the inside of the mold with talcum powder or cooking oil (I prefer olive oil), and fill the mold with the putty. Let it harden for a day or two and remove. You might have to do a little trimming with an Xacto, but you have the basic shape.
If you copied the existing arm, you have to invert the hand so it mirrors the one on the other side. Cut the hand, pin it, and turn it upside down, and glue in place.
If you are not happy with the casting results you can add to the detail by sculpting with epoxy putty.
I have done this broken legs on horses, weapons, heads and other parts. It works pretty well, but you might not have success the first time.
Hope this helps.