For your start and end screen, add new layouts to your project.
Make sure to organize them in the project bar so that the start screen is the first on top of the list. You can also set it in the project's properties as the "First Layout" of your project.
Related actions are system action "Go to Layout" / "Go to Layout (by name)" to move to those layouts.
I guess to spawn the Treasure, you need to add sub-events to your event 17 in order to check the type of object (since you want a specific "Manowar" ship only to trigger the treasure). When this specific ship is destroyed, then spawn (possibly use the system action "Create object") to place your treasure where you want to spawn it.