Mine was exactly the same when i changed my main jet, just do as blue says, use some gasket sealer and put an even smear over the area where the gasket goes then fit the gasket in, the gasket sealer will hold the gasket in place. Put some on the cover side where the gasket goes in and reassemble it once it goes tacky.
Same as Blue i did it maybe 6 month ago and its still fine now. I couldnt find a new gasket either.