Just a follow up to this.
First, I don't have the C64 WiModem from CBMStuff (Jim Drew's modem). But I do have a modem based on the ESP8266 WiFi module.
I have modified my modem based on Jens' findings and have some small points to share.
I initially tried going with Jens' recommendations and used a 330μF 16V Cap and a 330uH 190mA inductor.
That removed most of the noise, but not all of it.
I then set this up on a breadboard for further testing.
I did some rudimentary (amateur) measurements and found that the Wi-Fi module is even more power hungry than I thought.
Now I don't have a lot of parts lying around, so next I tried the same Cap, but used a 330μH Inductor rated at 330mA (because that is what I had available).
Result=all traces of Wi-Fi noise is gone.
I did run into some problems along the way. At first the modem seemed to be completely dead after I modified it. I tried to re-flash the firmware to no avail. In the end I re-flashed it to factory settings and then flashed the C64 WiFi modem firmware, and that brought it back to life.
Most likely this was caused by me doing something stupid.
Anyway, I now have a modified ESP8266 based Wi-Fi modem connected to my C64 Reloaded Mk II and no interference in sight.
Picture of mod included. Cap is on wires because I wanted to keep the modem in the case-