955i Engine Swap Issues

Favourite is the clutch switch, caught me out on my Daytona after I’d put it back together.
If it’s not that, are you getting fuel pump priming, neutral light on and no EML?

 

Got a multimeter?

Switch on and check for battery power from a coil +ve terminal to earth. That will tell you your ECU is live and the ignition circuit is probably healthy I.e, sparks.
Now check your starter circuit. There are three safety lock outs; neutral switch, clutch switch and sidestand. They all immobilise power to the starter motor. Then the starter button connections to the starter solenoid that should audibly click when energised.
Verify that lot first! Elimination tracks down the culprit....

 

Does the ECU care that it's a different engine? I'm not sure but I know some bikes do and some don't.

 

No, it has no way to know (or care)

The most likely reason for non-crank is the clutch switch, as previously suggested
You can use one big clue:
If you press and hold the start button (WITHOUT operating the clutch) you should see the headlights go completely off - and come back on when you release the lever. If you confirm that, it proves you have complete circuit through the START switch itself.
The same signal that controls the headlight-cut relay also directly feeds the positive coil of the starter solenoid; but the solenoid ALSO requires the clutch switch to be closed in order to complete the negative side of the circuit.

So - if your headlight DOES respond to the start button, you probably have a clutch interlock issue.

You did say everything else looks 'normal' and presume you have Neutral and Oil lights on the instrument panel, which eliminates any issues with mixing up the bar switch connectors (yes, they CAN be interchanged!) or the Alarm bypass connector missing. But if you DON'T have those indicator lights, visit those connectors.