Hello, Dave:
You don't have to replace the manifold. The heat riser valve may only need some liquid wrench or high-temp lube to loosen it up. Or the coil spring needs to be replaced. It does come out of the manifold, but you best remove the manifold from the head for easy access to the spring behind the manifold and to tap out the valve shaft. Be sure to index the valve plate to the shaft before tapping out the shaft so it will operate correctly after re-installation.
Best,
Ron Kurtz
E #292