As usual, I cannot be responsible for any damage to your machines when you will experiment the following tutorial.

This is from the Apple Web Site that was  found by bradleydad1 a recent eBay customer :

PowerBook G4: Keycap Replacement

This article explains how to replace a keycap on the keyboard of a PowerBook G4 computer.

Important: Shut down the PowerBook G4 computer before replacing a keycap.

There are four steps to putting a keycap back on the keyboard:

1. Check that none of the individual parts is broken. On the keycap, all of the snaps and hooks must be present and not bent or deformed in any way. (See Figure 1.)

   
   Figure 1 Snaps and hooks on back of keycap

    

2. Reassemble the scissors (plunger mechanism), if they have come off the keyboard, or are not fully snapped into place.
   • If the scissors are not properly installed, make sure the pivots on the ends are present and not broken off or deformed. (See Figure 2.)

     
     Figure 2 Intact scissors mechanism

      

   • The two pivots should still be snapped into the metal plate, and the bar on the other half should be captured under the hook in the metal plate. With the end of a small screwdriver--or something similar--check that the two ends of the scissors move freely up and down.
     

      

   • To fit the scissors into the metal plate:
     1. Orient the scissors relative to the metal plate as shown. Rotate the scissors, relative to each half so that they are approximately 90 degrees to each other. Slip the bar at the end of the inner scissor under the hook on the metal plate. (See Figure 3.)

        
        Figure 3 Scissors mechanism with inner scissor under hook on the metal plate

         

     2. Tip the scissors to one side and insert one of the pivots into the appropriate metal tab (while keeping the bar under the hook). Then gently push the other pivot inwards so that it can be slipped into its own metal tab. (See Figure 4.) The free ends of the scissors should move up and down freely while the scissors remain attached to the metal plate.

        
        Figure 4 Scissors mechanism attached correctly

         

3. Reattach the stabilizer bar (found on larger keys only) to the keycap if it is not properly installed on the key. Make sure the stabilizer bar is snapped into place along the bottom inside corner/edge of the key. (See Figure 5.)

   
   Figure 5 Stabilizer bar on back of keycap

    

4. Align and snap the key back into place.
   • For normal sized keys (roughly square) and smaller keys (function and cursor keys): orient the key to the keyboard. When the key is correctly aligned with an even gap all around (relative to the surrounding keys) press down with even, firm pressure in the middle of the key. It should snap into place and work correctly.
     

      

   • For larger keys with stabilizer bars: rotate the stabilizer bar out approximately 90 degrees from the key and insert the ends of the stabilizer bar into the tabs on the metal plate. (See Figure 6.)

     
     Figure 6 Reattaching stabilizer bars to keyboard
     

     While keeping the ends of the stabilizer bar in the tabs, rotate the key flat and parallel to the other keys, and align the key relative to the other keys, with an even gap. Then press down with even, firm pressure in the middle of the key. It should snap into place and work correctly.