Two ways.

#1 fake the whole thing. Render a potential continuation by simply shifting the camera to the left and then combine the images in Photoshop. Maybe as variation, render foreground of the original as separate layers and then change only the background image to the size you need. And again, combine that in Photoshop.

#2 change the ratio and then use the original image (with in Photoshop roughly added left side) as background so that you have a reference where your foreground must be. And then do the modifications so that it is what you need.

And always - in illustrations, render layers. More flexible. When the client wants to change more things, add whatever to it, you don't have to render the whole thing again.

One day your ship comes in - but you're at the airport.