Have a look at the waveform with an oscilloscope. For the cost of 2 sensors you could buy a basic oscilloscope on Ebay.
Consider something like this http://xj900diversion.free.fr/bus/I2C%20-%20RS-485%20adapter.htm
The parts should only cost a few dollars and a breadboard will tell you if it's going to work.
Arduinos are a minor cost compared to the sensors. How about adding one next to each sensor and then using the Arduino PWM outputs to transmit the readings. If the cables are very noisy then consider building a current loop driver circuit. For an extra $20 an Ethernet board will give you > 100m over cat5 cable.