Is the piece missing completely or do you have any pieces of it? If so that’s a head start, otherwise…..
Reconstruction of that piece isn’t impossible but it’s going to be a fair bit of work. It doesn’t look like there is internal detail to worry about.
I would use sheet styrene, maybe 30 thousands thick (max), shaping one side to match the break. Then I would score the outside of the piece with multiple lines parallel to the bend, slowly bend it over to close the gap. Once this is fixed and dry, you can build up additional layers of styrene to bulk the piece out.
Once it’s a bit thicker than needed, you can sand it to final shap and scribe in any details needed.
Hope this is enough explanation to enable you to fix this up.