I run that same engine and carb set up. I had to make my own intake manifold. Actually my friend Turkman made it. I don't know of any store bought alternative. Maybe AGK? But they mostly have parts for the GX engines.
Other than that, maybe contact member FFV8, really nice guy. I believe he was offering home made intake manifolds for the 79cc engines, maybe he could be persuaded to make one for the gc160.
Only other option I can think of if you don't have the fabricating skills is to get some white paper, hold it onto intake port and rub along all the edges, inside opening, bolt holes, outer shape. Then carefully cut out the shape to make a template. Do the same thing on the carb, take both templates to a welder and have him make the two flanges then weld them to a small elbow tube. Just make sure to orientate the flanges correctly to keep the carb level upon installation.
Hope that helped,
Gilbert