like said above, use your car's harness from the ECU to the shock towers, then the engine's harness from the shock towers to the engine. Then sell the OBD2 ECU and pick up an OBD1 P75.
It also depends on which HB you have. CX/DX/SI/VX. they all have a different amount of wires run in them all ready. the CX will require two wires to be run from the ECU to the engine harness at the shock towers. the others should have enough wires to make everything work.
It's not completely plug and play, but not far from it.