Never found it necessary to use shielded cable if the noise suppressor plug was used and eliminate the steel throttle link.
You've kept radio gear isolated best you can so I would try that first.
Shielding would be a final application if all else failed.

TV Co-ax will not be suitable as it is designed for TV signals at very small voltages, maybe just a few millivolts.
Ignition systems operate at thousands of volts and the co-ax does not have a high dielectric strength. i.e. the spark will jump across the insulation to the outer shield under load.

In days gone by, the co-ax cable had a fairly loose shield that could be removed quite easily and re-applied to another cable but these days, co-ax is quad shield and the shield is moulded in the cable and would be impossible to remove as once-upon-a-time.