I have finally got some time over for soldering the RetroKit from RetroZone! My thought was not to do too much modifications on the outside of the NES and therefore I desided to use the standard NES controller connections so all the modifications stays on the inside. I used the wires and the connections on the NES moder board to connect to my RetroKit. I still have the second controller and finding a USB socket to do so I can connect the controller to my moder board.
Note that the purple and the blue wire are not used and it is no need to connect those pins to the connection board. You can find the wiring diagram for the RetroKit here.