I had a Metrolux-II with the sensor and now have two RH Designs Stopclock Pro's, one is the compensating type with the sensor. I believe that the two units used basically the same type of sensor, so ordering a sensor from RH Designs through Second Hand Darkroom supplies might get you what you need.
I would also email Richard Ross who is a member here and ask him as he designed all those goodies and might know of the very components you will need or will be able to tell you if you can simply use the RH Designs sensor with a minimum to no adaptation.
Another thing to consider is that although less than ideal in your case, the Zone stuff is no longer made and RH Designs stuff is. I am willing to bet that the Stopclock Vario is every bit as good if not better than the Zone stuff.
It's kind of expensive now but you have to weigh the cost, time, still being made or not benefit factor. I got rid of the Metrolux unit for this reason, opting for current technology that is still being made versus stuff that is getting much harder to find parts for or multiples of for standardization reasons...