The 3 Legendary Birds, Cresilia, Rotom, Regigigas, Gratina, Mesprit (and maybe Heatran) can all be caught again using the wild pokemon modifier. Obviously, you need to at least be in the right cave, building or grassy spot so when you do catch them, their info is right. You'll also want to be sure to set their level up so you'll catch them at the same level as the event. You could get a lev 100 rotom in the Old Chateau, but it wouldn't look right.