If you don't have a socket quite that big you can use something slightly smaller as a 'dolly' and holding it inside the lip, tap it gently with a small hammer and it will round itself up again.
When sealing flanges we use black silicon roof & gutter sealer on non EFI stuff. It's cheap and works a treat and burns to a nice carbon gasket when it gets hot. Don't use glass and tile sealer as it has an etching agent to help it grip and will rust the steel away. It will have a vinegar smell.
