There's a replacement motherboard which uses an FPGA in lieu of an original GB/GBC/SGB SoC, so you can in fact build a Gameboy of Theseus with zero Nintendo parts.

https://www.youtube.com/watch?v=Cesh5OAL4uk

You can also get an FPGBC which is FPGA of a GBC. They even sell them prebuilt for $99. So all parts are 100% brand new.

https://funnyplaying.com/products/fpgbc-kit

The Game Boy of Theseus