I would say not to mutilate your Bonesinger to make him fit as an Archon.
First off, the Bonesinger is standing on CWE wraithbone - which would look wrong as just scenery under a DE Archon.
Second, the Bonesinger is wearing that funny wraithbone armor that all Farseers and Warlocks have - not only does it simply look like CWE psyker-armor, but those are CWE rune-shapes in the armor. You'd need to modify that to make it look DE.
Third, you need to cut the wraithbone triple-handaxe-thing out of his right hand and pretty-much replace the head (with its wraithbone antler-things), too.
What the others have hinted at and is the general norm is simply to look about the DE, CWE, and WHFB (fill-in-the-blank) Elves for bits you like and then do some kitbashing. I suppose you could resculpt a little to make an old Malcadon Spyerer or Imperial Assassin work, too...
The general guideline here is that you're looking for smooth-and-sinister armor, nothing that looks too crude or 'nice' - while avioding having the iconography of a different faction on your Dark Eldar.