I've seen on other forums, that a K100 dual output coil would work quite well as a substitute for airhead bikes with electronic ignition systems .
Don't know if you any motorcycle salvage businesses in your region that you may be able to get one of these, for substantially less cost, but you would need to fabricate an adapter bracket .
I have a Dyna 'brown' coil, quite satisfied with it .
The business that I got it from, Motorrad Elektrik, the owner Rick Jones has said that in the time he has been selling them, he's only had one or two returned, that had actually failed, average expected service life is 20+ years .
What sort of problem are you having with the coil ?