There are two ways I can see interpretting this stunt. The first (which I think is correct) is as described by Tsunami. To simplify (at least in my mind), you take the shifts you generated on you defense, and those become the shifts for your riposte attack. One oddity I note is that if you used full defense, you have to deduct that bonus from your riposte shifts. So what if your defense roll (using full defense) was just barely enough (0 or 1 shift). In this case, it seems as though you are allowed to make a riposte (successful defense) but you will end up with a failure on the riposte (-2 or -1 shifts). Yet the power says the attack is automatically successful. Which would mean that either you subtract from the base weapon damage, or such a riposte is treated as having zero shifts.
The other interpretation of the stunt, by the way, is to say that the riposte attack -- which is automatically successful -- is just barely succesful, so 0 shifts. Ie, you do base weapon damage only, regardless of the rolls. I don't think this is the right interpretation, since the rule about subtracting off the full defense bonus would be irrelevant if so; the inclusion of that rule implies that the shifts generated by the defense matter.
As for the rest, it looks to me as though any stunts/powers that improve your Weapons defense roll (except those that modify full defense) would benefit the riposte. In addition, anything that improves damage for the weapon being used would increase the damage done. Stuff that modifies your attack roll wouldn't, since you never make an attack roll, and any bonuses to full defense would be subtracted out along with the full defense.
As to True Aim ... it's a little unclear. The wording on the power implies a bonus when attacking to me ("When swung in keeping with its purpose"), but its a bit ambigous, and I could see it being rules as working for parry attempts, too.