Rather than hassle with replacing the gasket, I'd use a brush-on sealer like Copper Coat or Permatex Super 300 between the main cap and the old gasket. You're more likely to cause more leaks the further you tear things apart, and I doubt that the rear timing plate gasket can be found anywhere other than in a full overhaul gasket set.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!