I accidentally killed the Wraith of Sul-Senipul without taking the bow and then left for more than three days. I am playing on the Xbox and I have no previous saves. I guess I'm screwed now.

Well, if you want to finish the main quest there's always the back door - obtaining the necessary items, preparing for the confrontation, skipping all quests but the last one and going directly to the antagonist. But that kinda defeats the purpose.

I'm gonna see if I can edit the save game with a save game editor and give my character the bow.

Well, if you're on the PC, then just open the console and type:

player->additem bonebiter_bow_unique 1