Time to invest in a new one.

Sorry, I don't understand.

I just want to test the clutch before installing the engine back into the car.

If the engine is out you can easily pop the cover off and have a look. Removing the flywheel and replacing the clutch plate is cheap and easy and a decent investment while the engine is out

If the clutch arm is at about 90degrees from the line of the plunger, then likely there is significant wear in the system (left side of the photo). At this point the slave cylinder simply doesn’t have enough travel to release the clutch

I replaced:
Clutch arm
Plunger
Throw out bearing
Clevis pins

And you can see the improved angle of the arm on the right side of the photo

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All parts have been checked and replaced if necessary. I would like to have the clutch disengage with the motor out of the car.

In that case, get a long pipe/breaker bar and pull the clutch arm out until it releases

Or add the slave cylinder and use a pedal box to push the slave

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Thanks Timmy, that worked.