I'm not an electrical guy, but here's what I've come up with so far by virtue of the Factory Service Manual.
I'm assuming it's a non HISS bike.
Some countries got Hornets with HISS, but not the USA or Canada.
Does the engine crank?
(You're clear that the fuel pump doesn't prime, and there is no spark.
I realize no spark usually means it cranks but doesn't spark, but this is not time to make such assumptions.)
I'm assuming there is no PowerCommander involved.
IF there is, given your present difficulties, disconnect it to make sure it can't be part of the problem, whether it is or not.
The steering head switch and lock is not part of the ignition system.
It does not have a diode in it as best as I can tell.
If you have not already done so, see FSM page 17-0 and familiarize yourself with all the switches involved.
One or some of the switches, and/or their wires could be involved and need to be proofed as good or bad.
Also note there is a Diode in the wiring diagram. (I don't know if it's a Zener type or not, noting that Zener type is special and not a basic simple diode)
See FSM page 18-0 which is the Starter System pictorial and wiring diagram.
See how the "diode" is now called the Clutch Switch Diode.
And it is the same diode as referred to in 5 above, the wiring diagrams make that clear.
See FSM page 18-11 for what little info there is on the diode.
It's a module that is resident in the fuse box covered by the right hand side cover.
There a simple proofing test described as well, and the proofing tests suggest to me that it's a normal diode and not a Zener type - but I could be wrong.
I assume the lock assembly has been ruined as beyond repair.
And you Ebay purchase was for the entire core, complete with its key?
That you have/had to fit into the upper triple clamp after getting the original out?
Have you checked the contacts of the new core re continuity?
Scary thought, maybe you got a HISS type core, noting the ECU has to be HISS matching.
I'm pretty sure that a HISS type core assembly will not, and can not be made to work, on a USA 919 unless a HISS type ECU is also fitted.
I hope something in the above is of help.
Plus it's a decent platform for genuine electrical types to work with.