The safety sensors are not resistance-based, they work off a digital signal. That will be an almost impossible situation to bypass or override, since to meet the new code, manufacturers are required to make the units so the sensors cannot be bypassed. You will, unfortunately, need the sender unit and receiver unit I know because I had a similar experience with someone else trying the same.
I did not think the sender and receiver were that expensive, especially since a whole garage opener is only about $150, including sensors.
Either you will need to buy the sensors, or try to return the mainboard and just buy a whole new unit.