Yes, in general, I think most of the older fighters would translate well to the fight game today, and especially greats such as Greb and Langford.
Both Greb and Langford were pressure fighters that worked on the inside and with boxing less prevalent in gyms around the world, they had far more experience in training with other fighters and experience in the ring in their day. Through the gym wars, they learned how to absorb blows and shake them off because sparring was intense. They also stayed in shape year-round to be able to take fights whenever they came up and to hone their craft. Not to mention, they had same day weigh-ins and could not put on 10-15 pounds like middeweights routinely put on today before the actual fights. JCC Jr. was probably around 190 in his fight with Sergio when they stepped into the ring.
Greb on the inside would roughhouse and give a body beating to Sergio Martinez. Sergio would have landed shots, but he wouldn't be given time to reset and rest after he landed. Greb would use that time to nail Martinez and compensate for his slower hand speed. Greb was much naturally stronger than Martinez who is really a small middleweight/jr middle. I see Greb winning via a knockout or comfortable decision.
Langford and Froch would be no contest. Langford has faster hands, better boxing ability, and a wide variety of fighting styles. He could punch, feint, defend, or fight inside and outside. Carl Froch is a low economy fighter and reckless at times with indifferent defense. His handspeed is not very fast (Andre Ward was openly saying that to the media after his easy unanimous decision win over Froch) and while he has the power to land a KO blow, Langford had a granite chin and took bombs from heavyweights that outweighed him by thirty and forty pounds at times without going down. Suffice to say, I think Langford would win this fight in any fashion that he pleased.