If you're making a portable you probably need something to watch it on. (Unless you want to guess what's happening in the game, but I wouldn't advise that) Anyway, this forum is your "Hacking a pocket TV/screen" one-stop solution. Share your experiences and knowledge here.
It depends on the chips controlling the screen. If the chips are capable of taking RGB in, you'll likely have to solder directly to the chip taking the analog in to add RGB lines, as well as telling it to take RGB input, usually done via shorted connections (break composite configuration traces, and solder together the connections that tell the chip to process RGB).