Newton Adventure is not a game engine, but his code is opensource and not hard to understand. It is therefore quite easy to create his own levels/mods/games using it.
The first step to create a Newton Adventure derivative is to get the sources and compile the game and the level editor. This procedure is documented here.
Then one need to replace the game data (graphics, sounds, levels). The data structure is explain on this page, but the simplest way is to edit the existing data: images and sound can be easily edited using standard tools, but the level (*.tmx file) requires to use the existing level editor.
To test you can use the following procedure.
As sample, I use graphism from opengameart to create a simple game bonhomme aventure: