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A .bitsy file is actually just a text file. Go to the page of any Bitsy game, right click, inspect element and search the HTML for "exportedGameData". You'll find a massive block of text and comma-separated numbers. You can copy that into notepad, then change the extension to .bitsy when saving. This will only work for vanilla Bitsy games, as modded titles tend to store additional needed scripts outside of this location.
Hope this helps.