Lots of non-OEM options. This is from the Omega Ignition web site:
http://www.stephenbottcher.net/~omega/faq.html /5 /6 /7 R90S R45 R65 until 1981 - 2 x old style single coil (rated 6V@1.2Ohm wired in series) or dual coils such as Honda Goldwing, Dyna green/brown/grey
R45-R100R from 1981 on - 2 x new style single coils (rated 6V@0.6Ohm wired in series) and BMW 2V stock dual coil (rated 12V@0.6Ohm) or dual coils such as BMW 4V Dual, Dyna Blue, Andrews 6377B
The older points only ignition required the higher ohm cois - the 1981+ electronics can handle the lower resistance coils but will also work with the higher resistance coil.
Personally I'm carrying an old Goldwing dual coil as a universal replacement. Picked up two used ones from ebay.