I make my own extended mixture screws. Get some brass rod and a brass nut that you can make fit the rod pretty close and solder them to your stock screw. Measure the length you need to extend it, carefully. I use the "Little Torch" that I use in my jewelry work and "hard" silver solder. The hardest part is keeping it straight. A jeweler shouldn't charge you much to do it, if you supply him the parts ready to solder. Here's a pic of the one on the Transalp. And here's the one on the DR350.