Interesting. Think you might mean #43 in the BRP parts diagram (using 1990) -- appears to be no corresponding identification for the part. It is mounted between two carb gaskets in the diagram.
OEM 1990 Carb kit 0439073 comes with two gaskets with different diameter holes (carb to manifold), as well as what I take to be a leaf shim. It's probably a kit for both the 9.9 and 15 If you have the kit or parts of 9.9/15 kits, pick a gasket with the smaller hole and use it as a template for the restrictor you want to make.
I'm not sure why you would have any fuel leakage with a gasket in place, though. I would guess that if you have seepage, you will have poor running caused by an air leak.