Cork is actually decayed tree bark, and WHY on EARTH, somebody thought that would make a usable gasket material amazes me. If you research the archives here you will find thousands of posts to read about Viton, Nytril, Neopreme, and probably other much more reliable products to make a durable leak-free long lasting gasket from than cork.
And as for gasket sealants, I've had " So-So to Poor" luck with Permatex Aviation Form-a-Gasket. Use HYLOMAR UNIVERSAL BLUE gasket sealant. If it's good enough for NASA, Rolls-Royce, NASCAR, and ALL the Formula 1 race teams it will keep your new home-made Nitrile, Viton, Neopreme gasket secure and leak free.
Back in the 1940's, '50's, and '60's the valve cover, oil pan, and intake to valley cover gaskets ALL made from cork. ALL leaked. Now days they all are designed to use formed O-ring style seals, or in the case of older engines, thin narrow stamped steel outlines of the sealing areas are encapsulated with an elastomer, yes, Nitril, Viton, Neopreme, and become an ever-lasting seal.
I thought the last topic about those seals Gary Ide posted about the OEM gasket was a composite, cork impregnated with an elastomer and pressure sensitive adhesive applied. That should be a reasonably durable gasket.