A very small amount of RTV will work, or you can even try ordinary printer paper. I have heard of both working very well. Alternately you can always try a local motorcylcle or lawn mower repair shop and ask for a sheet of the very finest gasket material they stock. This is the route a lot of YS .61 fliers went when it was found the stock gaskets were not punched incorrectly and could block the passages in and out of the regulator body.

Good luck,
John