i needed to use 2 Exhaust gaskets to fill the difference in size.
That's the downside, the only way i know how to do it too.
You may can find some silicone tubing for planes at the LHS and cut it to length.
That is the only gripe I have with Picco, they use a small smooth exhaust port
on a lot of there engines. Gaskets can be a pain.
The big red Ofna 1/8th exhaust header gaskets may work with a smaller 1/10th gasket inside it. Silicone them together.