Bucky lets a small smirk cross his face and says, "well you were a stubborn ass then, and that ain't changes a whole lot. But I'm stubborn too, and I know that I'm your best bet." Besides, times change. But the one thing that doesn't is Bucky wanting to help Steve out, no matter what the problem is.

"Look, I wouldn't be just doing you a favour. It's been a long time for me, and you don't really forget what it feels like to get a knot into an omega. And you don't have to worry about bonding, since I can't do that anymore anyway."

To Bucky, it all makes sense, but it really is up to Steve. Maybe he'd prefer a different alpha or something.