Unfortunately ROMs are tied to radio firmware. For example I am very impressed by a CyanogenMod based ROM called CM-Minimou, which is a stripped down version of CM6. It has worked flawlessly for me for almost a month now, without any issues or Force Closes, other than the Angry Bird Beta does not work on it. However, it is build for the old radio version (3.22), though it will work with some issues with 6.35. I am not sure if there are any 2.2 based ROMs for the Magic that are specifically built for the latest radio Rogers introduced.
Originally Posted by crankpusher
To run a custom ROM you really want 3 things (a compatible radio version, an engineering SPL, and a custom Recovery ROM). The radio version is fairly straight forward to change. The recovery ROM is not bad either. You can brick your phone if you mess up the SPL so I always try and avoid being a trail blazer there. Best solution is to look on xda and find a detailed how-to thread that applies to your particular build/configuration.
You certainly can revert to an older radio. Links are included in the install instructions for the ROM...
bj - HTC Magic 32A - Rooted - CM6-Minimou [2.2]