You should be able to get replacement gasket from a woodstove store. It's a rope made of spun ceramic. As for the firebrick, if they are mortared in place, they'll need to be repaired or replaced. If they're just set in place, I wouldn't worry too much about the cracks; they're no different than the joints between the bricks. As for mortar, it's a special fire-clay mortar; you don't want to use regular mortar.

Check with your local environmental or air quality agency. In some areas, they have incentive programs where they will subsidize the replacement of a woodstove with a cleaner burning model.