Hi - Assuming the load cell is a strain gauge, all you will need are 4 resistors and a potentiometer in a bridge circuit. The values will depend on the strain gauge resistance and load range you want to simulate.
If the load cell is not a strain gauge then you need to determine the load cell output signal and simulate that. (it could be voltage, current, duty cycle etc).
If you need high accuracy, use switched precision resistors instead of the potentiometer.
Ron