I don't believe it is the engine causing the interference because I have quite a few hours on it, and has never been a problem. However, as you say, the ignition has to be eliminated so I will try your aluminum foil suggestion. The conversion is a RCIGN which are a better on interference. The plug is a resistor type, most definitely.
You are correct, it may not be the ignition. Where is the ignition battery located? Is the receiver, servos, switches and battery at least 10" back from the ignition and it's battery? Is the ignition switch located near the engine? Is the throttle push rod non metallic?

I got thinking over lunch and realized I used a metal clevis to connect to the tail wheel metal control arm on the pull-pull steering system. Could this be a contributor? I don't know why I did that, and it's buried inside tail.
If the pull pull system is in a little tension then the metal to metal contact should ok. I doubt that this is your problem.
Are you testing with only the top center wing section mounted and the rest of the wing sections not on? Do the center section wing braces connect to wood or metal inside the plane?

On my plane all of the braces and flying wires use DuBro rigging ends with 4-40 screws to clamp onto the strut fittings thereby providing solid electrical contact.

My name is not Bill.

Good Luck