Hypothetically if you managed to find a pin-out diagram for the emacs internal monitor connector and a diagram for a standard VGA monitor you may be able to splice the internal cable to a normal VGA cable.😀
Though the emac would then be useless and their is a chance you could bork the emac's screen. You could also accidentally damage your G5's video card if you do an incorrect splice😱 .
It would be more advisable to either buy a cheap LCD and use the emac for something else or sell the emac and use the money for a uber nice 20" widescreen LCD.