Unlike the other iBook models the 14" 1.42GHz and the 12" 1.33 G4 iBooks have a different setup for the magnet and the reed switch sensor that triggers the sleep mode. On these models the magnet is screwed to the middle left side of the the display and the reed switch board is located on the optical drive and it's cable connect to a socket near the keyboard socket on the logic board.
I think that you forgot to take off the screwed magnet from your broken hinches assy and installed your new hinges assy without the magnet. Check your broken hinges assy if you still have it, unscrew the magnet and reinstall it in your display.
To test the sleep function, find a magnet and pass it over the right side of your iBook just over the optical drive. Your iBook should go to sleep if the reed switch board is well installed and the cable connected to the logic board. Then pass over the magnet (or a metal clip) on the right side of the lid (about in the center over the bezel) and if the magnet is installed the magnet or the clip should stick to the lid.