Keyboard replacements in the Retina MacBooks are a bit of a beast. Unfortunately I can't find any 13"-specific chatter. Here's a (rather scary) [https://www.youtube.com/watch?v=dU48RuxReqQ|youtube guide for a 15" keyboard replacement], I say scary because it looks like the battery control board gets a little banged around (and be sure to remove the screws he forgot). With some scouting you might be able to see how well the process will work on a 13".
There's also an [https://www.ifixit.com/Answers/View/134399/Replacing+only+the+keyboard+without+full+Topcase|Answers thread here] that talks about the replacement in the 15" and mentions the 13" as well. The other challenge will be to find the replacement keyboard by itself.
Hopefully that information is of use to you! Best of luck!