Exhaust Valve?

To get the actual plate inside the exhaust manifold out, you need to drive out the pivot pin/shaft. You then need to remove either the headpipe or the exhaust manifold to take out the plate, or it will be laying loose inside the exhaust manifold passage (which may be your current situation).

If the pin and plate are still intact, you can make sure the plate is in the open position and keep it in that postion using a piece of wire, a wedge, etc. That'd be the quick, easy and cheap repair method, but far from the best.