So, it seems that the most straight forward is to just drop in place new version of kernel but with that one extra config flag set. And, if I get pretty much like for like version of it - then the rest should not cause any other issues (I hope).
I've got some experience building kernels, but that was pre year 2000 and even then it was a bit scary going through myriad of config parameters and trying to understand what they mean...
What I am hoping is that there's some simpler recipe like in https://www.raspberrypi.org/documentati ... uilding.md
So, I would be very grateful if someone close to the subject point me to how to configure it to build 64bit version, how to add config parameter 'CONFIG_CFS_BANDWIDTH=y' in, ensure it is made with all other stuff original beta 64bit kernel is made and warn me if there are any gotchas in the process. Theoretically I should, after the end of the process, be able to just update image (which file?) and modules (where?). I'm happy to put time into trying to get it - but quick bootstrap of the process would be really appreciated

PS I do have Pi4 with 64bit Raspberry Pi OS and SSD ready for the job...