Look at your schematic.
Receive data is taken from the dicriminator output.
Look for the 3.3K resistor comming off pin 9 of the last IF stage. Connect a 1K series resistor from the modem RX input to the junction of the .001 and 3.3K. PTT is taken off pin 2 on the microphone connector. This method has been tested and works quite well on 440 MHz and 220 MHz. The mods haven't fully been tested on the 2 meter portion of the radio. The TM-731 radio is different enough to cause problems.