I'd have a go at it yourself,most keyboard assemblies are pretty self expanatory to dismantle/Rebuild,its the taking apart of the synth and its casing that will take the most time
the keybed assembly will usually be able to removed from the Synth main frame,theres usually only about 2 or so connecting cables on most synths from the keybed to the main Circuit boards so you shouldn't have to disturb the rest of the synth
Its probably just popped out of its retaining clip,(judging by the way the key is situated on the Picture its happened to me on one of my Korgs,if theres nothing rattling around inside chances are there is no actual damage,if you lived local I would come and repair it for you myself
Most keys have a lug under which the key is first placed(usually pressed down and then pushed upwards if you push another of your keys forward you'll be able to feel the play in it) and then there is a retaining spring which gives it the tension off the back of the key at fixes it to the key assembly,looks to me like the key has been knocked from out of that lug when your dropped it
If its under warranty then i would try and get it done for nothing,But not sure that this would be covered by the warranty,as its accidental damage rather than malfunction,plus Turnkey are a *beeping* nightmare with dealing with warranties/Repairs,I'd never buy anything from them again!
The way I look at it,these synths are built on a production line by people who have absolutely no idea how these things work,or even care so,so if they can build them,so can anyone with a decent amount of common sense/care,I've managed to restore 12 or so Keyboards in the past many of them suffering from key damages amongst other things,and I have no technical knowledge on the assemblies other than just diggin in and seeing how its constructed,most of its is straight forward and self explanatory anyway if your technically minded
If you've managed to build a modular yourself,Repairing the keybed on your Ion will be a walk in the park!!!
Just take some pictures of the assembly as you strip it,so should you forget how it reassembles you have a reference aid,and make sure when you asemble it you keep the screws seperate so you know how each set is reassmbled in turn,theres nothing worse than to find your a screw left over after putting it all back together!!!