It's probably very solidly glued in. So if I were you, I would probably buy an extra boiler shell to try this. You probably need to tap it from the inside, pushing it out. I suspect that some kind of extension like a long screwdriver with some padding on the end of the blade might be needed.
I haven't seen this boiler, but I suspect that trying to get a knife blade between the smoke box door and the smoke box would probably ruin them both.
I haven't seen this boiler, but I suspect that trying to get a knife blade between the smoke box door and the smoke box would probably ruin them both.