If you go to this thread - http://www.ww2aircraft.net/forum/tec...ons-21987.html
and scroll to the last post there is a partially dimensioned drawing of the KG13. I found it enough to work from, although the buttons don't seem to be of the most common type used on the grip.
Going by photos I'm not sure that they made any two KG13's exactly alike

Edit :- the forum seemed to be down when I checked the link, but it sometimes does go offline for a short while. If the link doesn't work just try again later.