Someone made a whole series on how to build a general pupose redstone computer. You can program it to do anything you want with already written programs like a calculator, connect4, a maze generator or even Tetris. https://www.youtube.com/watch?v=hAZEXqWLTmY

The cpu and mobo are one part, you cant replace just the cpu while keeping your old mobo. But every other part can be individually replaced.

You can see all replacement parts here: https://frame.work/nl/en/marketplace