It would be pretty hard to answer that question without knowing exactly what has been done to the saw, how radical the saw builders went.
I'd suspect that the saw would still be competitive, but would suggest that you keep it as a keepsake. A modern saw built on a big Stihl or Husky just wouldn't have the same "aura?" as the big old 125 Mac. It would be a shame to take a chance on hurting it in some way, even worse hauling it around, giving some scumbag the opportunity to "lift" it when you were'nt looking.
I was recently talking to an old aquaintance: this guy once raced anything that moved, but had a real affection for motorcycles. Two bikes were stolen from his shop: a yamaha RD-250 and a flat-tracker based on a bored and stroked Honda XL-350. They were obviously stolen by someone who knew about bikes because they hadn't been moved in 20 years, but the flat-tracker hurts me more because I built the last engine in it.
I'm not a violent guy but I'd like to see the thief fall down and get an owie.