All Titan's have the Titan's Core in common. This is a glowing orb precisely 27" along its circumference. It is the power source for the Titan and prevents the need for any other organs. The destruction of the core is perhaps the most definitive way to kill a Titan.
In addition to that, most Titans over 15 ft have Titan Crystals. They are used to distribute mana throughout the larger Titans.