If its not cracked, then a decent cleaning in hot soapy water, (don't bleach it) followed by a good waxing should revive it nicely. Finally, use butyl tape when replacing. Make sure you work the butyl tape in your hand first to get it pliable, make a ring around the collar of the plate and offer it up in the same holes. When screwing it back down, go slowly and turn each screw gently at a time, this will avoid any cracking and allow the butyl tape to spread slowly, giving you a nice even clean seal. Please remember to clean the decking before doing any of this. I use a wire brush to clean it and then wipe it down with acetone.