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The best way to do the test is using a remote start switch.
Put one lead on the starter solenoid battery terminal. Then put the other lead on the yellow wire small terminal on the solenoid. This will allow you to crank the engine without the ignition. This prevents stress on the ignition and is safer.
You can also remove the cover from the powerpack. Locate the screw with the black/yellow wire. Using a small jumper wire run the jumper from the black/yellow terminal screw to ground. This will stop the ignition from firing.
If you put the spark plugs back in the leads you won't be able to properly ground them and in addition they will be near the plug holes which could ignite the fuel. So keep the plug wires away from the spark plug holes.
I also would disconnect the fuel line connector from the engine.
In 1977 OMC didn't have a emergency stop switch, however if you had one installed just disconnect it.
If you need further help with this let me know.