Exhaust flap valve

Not sure where I got it from but try this :-

You could remove the valve and soak it overnight in a bucket of diesel to dissolve the carbon deposits. Thats how we used to free up seized exhaust drain valves on minesweeper Deltic engines (2-stroke diesel with huge overlap).

It might smoke a bit once it gets hot afterwards but should burn off.

For lube, get some nickel grease, dont use coppaslip as it bakes solid.