Look in the service manuals!!!!
And there it is described how to hack into the ECU or tachometer-bus? Interesting
There is nothing at the
ECU-Hacking Forum, which is always the first attempt for searching something like that.
But they lead to Magneti Marelli as the ECU manufacturer, which may give you a hint.
Tuning is possible as you can see
here, so you will have another idea of connecting to the Harley´s ECU.
Personally, I only know about Denso ECU´s from Suzuki and Kawasaki, where are plenty of pages about here in this sub-forum.
Good luck!