Just reduce R8 and increase R9 if you need more gain. The 741 isn't great, but the gain is in the tens of thousands - enough for an inductive loop detector.
Peak-to-peak is only going to be a couple of volts, though the 741 can't go rail-to-rail; it stops a couple of volts short of Vcc and Ground. Can you increase Vcc to 9V or 12V? You should see better results with that arrangement.
At some point, redo the circuit using an LM6132 amp. Different pinout, but much better specs. The output goes rail-to-rail, so you get the full 0 to 5V. Better gain, better impedance, and frequency response.