With many threads on the subject, and am not quite sure where to post this - so thought I'd use this one :)
The 2013 F800GS uses a 43mm WP fork. Talking to the KTM guys and it looks like some of the older KTM's used a 43mm fork as well.
Would the WP 4357 (43mm) work and/or be a drop in? It apparently was used in a variety of bikes and had the compression and rebound settings (top and bottom) that the Shiver replacement for the 2008-2012 are taking advantage of.