Since the plug is intact - the blu/yel wire entering the gray plug from the passenger compartment connects to the starter solenoid spade terminal via another wire which is supposed to also be blu/yel. So just make sure that the starter is connected to that blu/yel wire at the plug with a wire of matching gauge regardless of its color but preferably blu/yel.

You have previously said that there was only one blk wire. That most likely goes to the oil pressure sender as we have indicated. Why would you assume otherwise?

The blu/yel wire to the injection comes out of a service socket. Which means it probably is there to give the guys on the assembly line something to do.

Continuing .. From the ignition switch a red wire returns to the engine compartment and connects to the terminal block beside the headlight relay. From the terminal block a red wire goes to the battery +ve and from there a heavy gauge red wire goes to the starter solenoid. That completes the starter wiring circuit.

So it appears, at the least, you need to join the blu/yel wire at the gray plug to the starter to complete the circuit.
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1980 245 Canadian B21A with SU carb but electronic ignition and M46 trans in Brampton, Ont.