The only way you can get your desired Pokemon, you first require an Flash Cart, like M3 Real, CycloDS Evolution, etc. or an Action Replay, which utilizes codes, instead of an save file, the Flash Cart is a much more simpler way to edit your Pokemon needs, but then you also must start a new game, because Flash Carts only support ROMS, so you actually have a physical copy, so i suggest to purchase an Action Replay, unless you don't mind spending a few hours playing Pokemon again.
Go to the request section of the site, to request for a Pokemon to be created, then traded.