I think that if you re a good man, a good model, example, you can raise someone else kid and this kid might end following you instead of his biological father. But it's very rare all of this ends well for everyone. Most of the time biological father will taint the mind of the kid, by jealousy etc, step father will feel it, will feel betrayed, will badly treat his step son/daughter etc.. Mother might protect kid and ex father...a disgusting mess, psychological games being played constantly...you have enough psychological games to win when building a family with someone who dint have kids with someone else...
If you raise someone else kid do it right and cut all ties between him and his father, and make sure you can be a better father than him.
Also first make sure the father is the only real clear responsible for the break up and following mess, and not the woman you re seduced by...or you re simply the next on the list.
all of this is not natural, is perverted, distorted, and it takes a very very strong mind, sould to handle all that shit and improve it instead of ruining it even more. This is why i say if you re not strong enough, do everyone a favor and go find somthing easier to create. Dont take the risk of being considered as the only responsible for everything that could go even wronger.
Who s the asshole in all of this? the woman? the ex-father? Certainly not the naive guy falling in love with the woman. Let them deal with their mistakes. In fact they should go back together and raise their fucking kid. If it's not possible, and if you take her, make sure she, her kid, and the ex father wont cause you any troubles. And get her pregnant with your own offspring.