I would echo that Kev, mentioned in the link to another thread, has done a bit of work getting this documented.
Another forum contributor, whom I do not recall the name, has said that he has taken his sensor apart and replaced the humidity sensor three times or so.
The Honeywell sensor is available from DigiKey and others, less than $20 the last time I looked.
On my board I had a thick coating of a flexible conformable water protectant. It took a bit of very careful work with tough very sharp small scissors to snip away to get to the solder points. The stuff was more like SprayLat on desert stored aircraft than it was the hard clear conformational coatings I've seen on other boards.
I am no expert on this, so TWI may have used different coatings at different times on their sensors. Apparently the sensors come with calibration points, or used to. The reason I'm a bit uncertain is there was a dealer in Florida who was selling out the stuff he had, and actually made an alternative board, one which uses a fixed voltage source feeding one of the newer humidity chips, and the factor there was fixed. I'm not sure if all the new sensors come with a much more predictable setting or not to enter into the 0% and 75% humidity calibration points on the TWI control boards.
Again, Kevin has some rough sketches of the boards and even some info on the alternative boards. I'm not sure if the dealer in Florida is still on line or not or has sold everything out. You might search eBay for TWI, and the last I saw he was selling a tipping bucket rain gauge from them along with a few other things. You could see if he still has a presence on eBay and contact him to ask if he has any of those boards left.
Good luck and to keep these old systems going, please ask if you don't get everything up and running.
Dale