I made a implementation of Deadfish in Nintendo Family Computer. You can download from: http://zzo38computer.org/nes_program/deadfish.zip It isn't very good (mainly because Deadfish isn't very good!), but, it is implemented. It doesn't implement key repeat, but it does deal with overscan (text isn't displayed in the leftmost eight pixels, and the bottom sixteen pixels of the TV screen aren't used). This program uses OAM in ROM, instead of in RAM. Probably this program could be written in much smaller ROM (even in the single-chip mapper it probably can be!), but 16K is the smallest ROM available so it uses that.