Exactly where is the it slipping and where is it worn? Is it the General purpose nylon arm that is slipping on the brass tube or are the soldered joints on the custom brass horns broken and slipping.
You are not going to find that exact thing except perhaps the general purpose nylon horns and all the rest are custom fabricated by the builder. Also to arbitrarily replace it, well that that is going to involve major surgery. I would be more inclined to simply find the problem with the slipping and address that.
Of course the last resort would be to abandon that crank altogether and go ahead an do it the modern way by using a simple double pushrod. What works very well indeed when double pushrods are used is to of course just have the wire portion of the forward end of one pushrod slightly longer before it goes into the carbon fiber or wood dowel and then the second pushrod at the forward end just has a few inchs of wire without anything. These two forward ends are next to each other close near the servo and two wheel collers are slipped over them to allow perfect adjustability but secure.
John