Is there a way to copy my entire game and use it as the basis for another game so that I wouldn't have to start the new one from scratch?

The simplest way would be to make a new project to generate the filepath/folders, then take the game you want to copy, save as, overwrite the newly generated project file, then change the information under the game object, setup tab. Most important would be Game ID - hit the 'generate...' button to the right of that field to get a new ID, as if you uploaded a copied game without a new ID it would cause problems.

This is what I've done before, anyway. I ended up taking the concepts I liked from my old projects, reworking the ones I didn't, and then constructing a framework with things like my combat system, resources, shops, etc all functioning and working and ready to build off of. A sort of engine-in-an-engine, I guess.

Thank you so much for answering! I don't quite understand what you mean when you say save as- I don't have a 'save as' button is that because I'm working on the web version? Is there any way to do it from there? (It's my first game so I'm not very familiar with the software or any of the code behind it)

Oh, I'm afraid I don't know a thing about the online editor. It's heavily recommended you use the offline program, unless you don't have access to it for whatever reason.

Simple.
Why not just save the game using a new filename.
Load in game A
then save as Game B
Remove the bits you don't want from Game B and you've got a workable copy of Game A to use as your new adventure.

On the web version, sadly this isn't possible.