I have had several magneto spark engines that drove me nuts, until I completely wrapped the system in metal (foil/braid etc) and used shielded plug caps + resistor plugs and earthed the wrap on the engine.
Take care that there are no "leaks" in the wrap through which RF interference can escape.
No metal connection/rod/cable anywhere near any ignition/engine component going to, or even near to a radio component.
Keep ALL radio components and any leads connected to the radio system at least 8" away from

That never failed to work for me.