Alternatives to the PS223 power supply monitoring IC
This looks to be a specialized IC that replaces lots of analog circuitry. Some simpler ATX power supplies use discrete circuits to generate the power good signal after a delay.



This example uses an LM393 comparator, for instance, to check the +5V rail and to make sure that OVP is not fired before allowing the power good signal to assert. In a pinch, you could try and replicate something simple like this.