Im pretty sure you could just open the world and edit from there…
Change the world size by selecting the Landscape Layer and changing the XYZ settings?
Not sure if you would be able to sell the game though (make money) as you’re using free content provided by the engine itself? Would have to talk to Epic about that.
But you should just be able to open the world and use it as your Persistent Level (add other levels onto the map by using Level Manager)
Let me know!