There are many examples of people retrofitting an old Mac with a Raspberry Pi. The one in the video above from jaromaz lets you upgrade an LCD screen with a Pi inside an old Mac shell. All the basic materials, including the screen, are changed in this way. You can keep the screen if it’s in good shape, but it’s very simple in terms of how you can change it. You’ll get better performance with a new screen, but you’ll have to deal with the original Mac screen with a CRT monitor. The 3D printed part will fix this for you if you are converting a flat LCD screen, but only if you have a 3D printer to support it.
Don’t forget to hack the software for Mac OS you want. Basilisk II is a popular alternative for Mac Os 7 and 8 if that’s what you want. Once this is installed, you will have a new machine that not only looks like the old Macintosh but also performs the same. For more information, jaromaz breaks down all the builds on GitHub.