Naphtha is what I use to remove old adhesives. Use in a well ventilated area away from sparks or flame.
I got a nice e-mail back from Morgan Sparks about getting a natural leather covering. He noted the problems about paring the leather down to an appropriate thickness and the issue with the grain not looking right with most natural materials on the Linhof, because of the number of surfaces to cover. He said that there was an appropriate ready-made leather that can be found on some German pre-WWII cameras, but he hasn't found a modern producer for it. I'm going to ask an acquaintance who is a book conservator if he knows of anything. He estimated $130 for an embossed leather covering and $175-200 for kid leather for the Technika.