If we use Rebuttal on an enemy Star-Crossed, can we choose a new target of that Star-Crossed on Ruin Runner?

It depends on whose Ruin Runner it is.

Your own Ruin Runner: Yes. When you take control of Star-Crossed via Rebuttal's [A], "friendly" and "enemy" re-anchor to you. Since you now control the spell, it's no longer an "enemy spell" to your Ruin Runner, so you can legally pick it as the mandatory "friendly unit" target (it would bounce to your hand).

Your opponent's Ruin Runner: No. Ruin Runner can't be chosen by enemy spells and abilities (Rule 758, 355.9.b; FAQ 8565), and Star-Crossed — now controlled by you — is an enemy spell relative to their Ruin Runner. You can't put it in the "enemy unit" slot, and it's not friendly to you, so it can't fill either slot.

One reminder: Star-Crossed's two targets are both mandatory. Even if you retarget onto your own Ruin Runner, you must still pick a legal enemy unit for the other slot (FAQ 12084).