I bought 2 second hand ones from Blatchat.
I've only used the one, so if you want to offer me half of what I paid let me know, I'll send you a blatmail once I dig out the price.
I'm not sure about the advance curve for a twin cam, but I'm sure if you found that out, you could return the dizzy to Aldon and they would adjust it to suit your engine. I'm assuming that if you have a xflow block it should fit without a problem.
Cheers