FunnyPlaying recently came out with an FPGA-based version for an affordable price. https://funnyplaying.com/products/fpgbc-kit?variant=40858870...
They also have a new, populated motherboard for the Game Boy Advance, into which you have to transplant the CPU and RAM.
It can use a clone CPU that was manufactured (until recently?) for the "GBBoy Colour" (and could also be pulled from the sort of failed SGB-like clone called the "Extension Converter for GB").
> I redesigned the Gameboy Color from the ground up
> based on the premise that it wouldn't require any
> components to be harvested from a real GBC, so it
> would be compatible with both the original GBC and
> clone hardware taken from a GB Boy Colour- since
> the KF2007 clone CPU from the GBBC is practically
> a drop-in replacement for the CGB CPU found in a
> real GBC- no software compatibility issues at all.