I've had several requests for information on exhaust gasket sealer.  I
use it as a gasket dressing and not as a "Replacement" for gaskets.
Permatex Ultra Copper RTV Silicone Gasket:

http://www.permatex.com/documents/tds/automotive/81878.pdf

There are other brands, too, and one calls their product "Mega Copper" RTV.

Of course, you first want to file the mating surfaces flat if necessary.

Mike