Right. In all cases, you would need to mod something. If you were able to channel the input from a previously saved replay to the opponent, you'd first have the problem that the opponent starts at a position different from the one at which the player starts and if you force it to be the same, you'd have instant collision. So say you were able to disable car-car collision, which probably isn't as complicated as it sounds, then yes, the original engine could allow you to play against your ghost.
Now, as Daniel3D pointed out, if what you want is to see your several replays playing together, you can channel them through my graphics engine and that is already feasible.
Now, as Daniel3D pointed out, if what you want is to see your several replays playing together, you can channel them through my graphics engine and that is already feasible.