Been watching this with much interest as was thinking of doing mine. But cable connection by coin door has been removed 
My update is also locked out, Think mine is on version 1.2 or 3 from memory have a couple of old laptops with the 9 pin rs port so thats a plus and somewhere an old keyboard. So hoping you all the best.
Yes my db9 cable was missing too, I'm currently restoring mine and replacing everything, and have removed all the cabinet decals ready for the new set I have.
The update was one of the first things I have been trying to do and I have never met such a pain LOL... Hope when you do yours you get an easy update.
Also during boot and in attract mode it tells you what version firmware you are on
I found my cable tucked away in the cab and the pc end was hanging off 1 of the cable clips.
