I wouldn't know what to do, but if you do not get any response here, you might trying calling some koi places. Keep us up-to-date.

That looks like a goldfish to me...not a koi. What do you mean it can only breathe from one side? It looks like it probably has a deformity of it's gills, which isn't terribly uncommon in a large spawn of fish. If it stays up right....and you don't mind the deformity...do nothing. It lived long enough to get colored up and to a decent size. That won't kill it. Either way, there is nothing you can do about it.