Spark Plug electrical noise

It may not be Rotax at all. Several times I have seen it being the aircraft wiring.

I'm with Roger on this one...  Do you have a USB power plug inserted into an AUX 12 volt socket?  If so, try unplugging it when the noise is going on...  Some of them are extremely noisy!  

If that's the source, replace it.  The USB plugs that Sporty's sells are not noisy, nor are the ones made by Anker (via Amazon).  I'm sure there are others, but those are the ones I've tested and found to be good.

First, determine the source of the noise and entry route.
If it truly is ignition noise, the pitch of the whine, buzz, static will vary in proportion to the engine rpm and stop entirely when the Mags are turned OFF.

Adding Filters and Capacitors is masking the problem, Not fixing it.
20,000uF caps will be effective in the Audio range, but will not affect anything above 100Mhz.

Next, how is it entering the radio?
Power Down/Pull Fuses for EVERY item in the plane except for the Radio,  Quiet?
Disconnect the antenna cable from the back of the Radio.  Quiet
Listen on a handheld radio in and around the plane.  Quiet?
Connect the aircraft antenna cable to the handheld radio.  Quiet
Disconnect the RF cable at the antenna end. Quiet?
At either the Antenna OR Radio back panel, Remove the connector and touch the Cable Connector's Shield/Ground to the Antenna Connector Shield/Ground.  Still Quiet?
Power the radio from an isolated temporary battery.  Quiet?