How hard is it to replace Keyboard on MacBook Pro 13" Retina (Fn keys)

Have a look at this Macbook Pro 13" Retina keyboard replacement - YouTube but I would try and replace the keys that are missing

While it is possible it’s not something I recommend for a first timer. This is a very nasty job as you need to be fairly physical yanking the old keyboard out and not damage other things in the process. Then you need to get all of the plastic rivets out and the replacement keyboard better come with the needed screws and do make sure you get the same exact keyboard as the ISO case is different than the NA/US case.

If the keyboard is too badly damaged and that is the only option I would recommend replacing the full uppercase assembly. While expensive it will get you to a better place MacBook Pro 13" Retina (Function Keys, Late 2016-2017) Upper Case also note you likely need a new battery anyways given the age of the system. So plan for that as well, as transferring the old battery really doesn’t make much sense as it likely will be damaged just in the process of removal MacBook Pro 13" Function Keys 2017 Battery Replacement

Here’s one source for the KeyCaps if that’s all you need 2016/17 MacBook Pro KeyCaps click on the image to drill down.