As someone who worked in that era, it really varied on how much support you would receive from the original authors. In this case he worked mainly from the reve

As someone who worked in that era, it really varied on how much support you would receive from the original authors. In this case he worked mainly from the reverse engineering of the data (WADs) that was available and used his own experience to recreate the game. When I was doing ports of games from Japanese companies or other people doing arcade ports for example often all you got was a copy of the game. I remember having to get so good at the game in order to be able to finish it to recreate it correctly! Best case scenario you get the original source code, art assets and audio. Worst case you have to work it all out and reverse engineer everything. Usually at least you would get some art and audio assets.