In Double Dragon 3 on the NES you can play as the ninja character. It's pretty close. I love DD3 as well but DD2 was the best of the NES trilogy. I loved this game so much. I agree I wish you could play this hack online. I wish there was a Mario maker version for the DD NES trilogy. You could make your own levels and enemies and play as any character in all 3 games. Being able to mix all the games and create new levels.
If you have enough hacking skills, you can change the hair of Billy and Jimmy Lee to the enemy ninja in cutscenes with the climbing sprites for the ninja edited from Jimmy and Billy Lee's climbing sprites as well. If you also have the text editing skills in video games, that can work. That way, you have completed your Ninja hack of Double Dragon II. Good luck!
