A1243 Keyboard space bar does not work

I have a Modern Apple Keyboard with Numeric Keypad (A1243)with all the keys working except the space bar. I removed the plastic key pad ensuring there is no debris underneath. With the plastic key pad removed, I pressed on the black plunger located in center of the area designated for the space bar. The space did not work.

Other than replacing the whole keyboard, is there a solution to replace what apparently is a defective sensor?

If so, how do I go about replacing the one sensor which the plastic key pad actuates?

WJM