Good footage of Oldtimers Jack Johnson and Joe Jennette "sparring" with Harry Wills as referee. https://www.youtube.com/watch?v=_7JxF01xjNc
He was a gamechanger, certainly. How you think that sort of style would fare today depends on what you think of the game at the time he changed it.
One of the people Johnson is sparring with if Marty " kid " Cutler. I think John L Sullivan worked with him. He's terrible in that sparring. His ring record? Box rec says 0-10, with 10 KO losses. http://boxrec.com/boxer/11653
Also check out trading reel number two...Jeffries and Johnson sparring footage not together but Sam berger and jeffries