You can do a real demake using GB Studio or programming directly in the language of the console, I use GB Studio it's much easier and faster, you don't even need to know how to program, you can also make games for GBC and if you want you can make a port with Goomba so you can play it on GBA or simply adapt it to the Snes peripheral (keep in mind that you have to respect the limitations of the console)
Viewing post in Samorost - Demake (GB And GBC) comments
It's actually very easy when you get used to the limitations, usually the limitations that cause me the most problems are the sprite limits or the music and sound, although it also happened to me that the gb and gbc color palettes don't fit well with the sprite, by the way, explaining all this made me want to do another demake or anything for GB and considering that I finish my GB games in 1 week or 2, it is very likely that I will