A modder was able to make a fully functionally N64 that was portable. With its look of the Game Boy Advance, this might be a dream for fans of the Nintendo 64 era. However, this modder, who goes by the name of Bacteria, released notes and videos of the construction of making the portable Nintendo 64, iNto64. You can see notes and videos of the page on his website.
It took the modder 4 months to build the iNto64. Here is a demo of the system: