Top Down Scrollers, Platform Games, etc. are all basic game engines that any beginner can write.
The 1980's games companies used to churn out such crap all the time.... all for kids with less intelect.
In all cases one simply changes the background, sprites, weapons atributes and health atributes to change the game into whaatever you want. However, it is always the same crude engine.
Even back in those days, I found "Scrollers" and "Platforms" insultingly boring and "samey", so generally prefered more mentally stimulating computer games that involved conquest, strategy, etc.