Very interesting - and I appreciate the time, skill and patience needed to do it all. A good job!
Just a though on the HT cable... I wonder if on a small engine you can use a smaller plug gap than on your car, and thus lower voltage - which will allow a smaller OD of HT cable? I suspect the outside of the spark plug can only resist flashing over a much reduced voltage compared to the "car" standard plug bodies. But the design always must be that the "weakest link" of resistance to arcing/sparking shall be the spark plug contacts inside the engine. So the outside of the spark plug shall have a higher flash-over resistance, and the HT cables similarly. I suspect you are using proprietary coils/ignition trigger/spark plug, but I am sure I have seen models with much smaller gear for the size of model?
K