It took nine or ten months from first having the idea to making a serious stab at realising it. Some of that time was spent learning the basics of Kicad, trying to get my head around water analogies about the motion of current through a circuit and feeling simultaneously exhilarated and intimidated about how much there is to learn. It's only in the last two to three months that I've dedicated my small amount of free time directly to making it.
A first attempt at redrawing the schematics I found online at http://retro.hansotten.nl/6502-sbc/osi-300-trainer/ led to a drawing I did not understand with over 100 DRC errors I did not understand either. I finally scrapped it and started again, trying to break the circuit down into sections I could comprehend, simplifying where necessary (such as only having one data switch) and only adding more sections (or the missing switches) once I was confident what I had was correct. This confidence was sometimes misleading and it wasn't unusual to discover that I had made mistakes but slowly over a couple of weeks a schematic emerged from this process that I think I understand and I think will work. Although I won't be surprised to find it doesn't.
The original OSI 300 is 10 inches by 8 inches with beautiful hand drawn traces and lots of empty space, at least as far as I can tell from photographs I've found online. I have never seen one for real. If things go very well, I might be tempted to try emulating this hand drawn look in a later iteration but for now I am just going to focus on getting the board to work.
I've preserved the ratio and used large switches and 5mm LEDs like the original but the board is smaller so that the unpopulated PCB will fit in an A5 envelope. This will make it easier to ship if I sell them to people who don't want to manufacture their own. I will also put the schematics and Kicad files up online once it's finished so people can make their own and or use it as the basis of their own projects.
Designing custom footprints for the switches I have found was fun. I am not 100 percent certain that they will work. just as I am not certain that the circuit is correct, but I have reached the point where I am willing to get a test PCB made so that I can add the components and start to debug any problems. I have sent the gerber files off to be manufactured and will update once the first PCBs arrive.