Perhaps borrow on Toni Clark's fabricated adapters. It should not be to hard to cut and drill a couple of brass plates to match your engine and carb inlet/outlet manifold shapes. Then select some brass or copper plumbing connectors the size of the carb (or engine) throats and hard solder or braze the shape you need. It may be that 1/2 inch copper pipe fittings would be close to the 10-12 mm size mentioned. Once you find that it will work for you, you could refine the design as needed. Should be an interesting exercise.

Sincerely, Richard