This stuff (as all of your things here) too has its source code downloadable.
This particular game is designed even easier to mod. That's why we called it SKIN-VADERS. :) The background and the enemies are all a simple iff(ILBM) picture. If you rename the enemies file to the the filename corresponding to an other stage ,than the enemies will appear in that stage.
Or you can draw a hole new background or enemies with Deluxe-Paint (or with any program you choose to draw). You doesn't even have to touch the source code. :)