Magneto engines normally don't need shielded caps..There are literally thousands of magneto engines out there with no shielding at all...A proper set up is the key...
There were no shielded 10mm spark plugs until recently when Champion and NGK came out with the RZ and CMR series plugs..There is no current shielded cap made for the 10mm Champion and NGK spark plugs...All my conversions that have the ignition inside the mount use standard unshielded spark plug wires and caps with a resistor spark plug..No one has complained about interference....
C&H ignitions are not inside the mount, so the long spark plug wire radiates RF..C&H shields the wire from the ignition box to the end of the wire, where it's grounded to the cylinder..The Bosch cap is an extension of the shield, completely cutting off any RF radiation...
Other manufacturers MUST have shielded caps because until recently there were no resistor 10mm spark plugs...
The Zenoah G26 uses a 10mm resistor spark plug with no shielded wire or cap..Works fine..
It's the only magneto engine currently made using this setup..
I once put a 10mm CM6 in a G26 because it's way shorter than the RZ7C the engines come with..Used a DA shielded wire and cap..Made the engine shorter from the crank to the top of the spark plug..Lost rpm because the shorter thread on the spark plug lowered the compression ratio.....