It's been a long time since I last logged in, but I think you can't write for pokemon that belong to the fifth generation. Or, at least, they are not in this
list, which is official. So, I am sorry, but I think your story can't get you anything. (Somebody correct me if I am mistaken). However, all you have to do is change the name of the pokemon you are going for and replace it with the name of one of an older generation. But keep in mind the list, because there are many levels of difficulties and your story may not be good enough if you pick a tough pokemon (I haven't read it, but I am saying just in case you go for, e.g, a porygon which is really hard to get, or something similar).