I finally finished replacing the entire housing (upper and lower).
It was a tedious job because I had to remove the two temperature sensors from the housing, insert the lower housing roughly in place, then reinstall the temperature sensors. There wasn't enough clearance to just install the lower housing and temperature sensors as an assembly.
Worse, when I removed the new sensors from the new lower housing I found that both sensor o-rings had been damaged when they were installed. Zabteck was very kind and sent me new o-rings, but when I installed them they felt like they would only hold a couple PSI, if that. I think I know why. Tight-fitting o-rings are easy to shear when installed, and I'm guessing Austekk found that the o-rings were getting sheared a lot and decided to loosen everything. In fact, the sensor seats are about 0.025" diameter larger than the original seats in the plastic housing.
I happen to have some thick rubber stock from Grainger, so I cut new tight-fitting o-rings with square cross section. They take up the play in the seats. I hope the rubber is resistant to ethylene glycol. I guess time will tell.
Other than the sensor o-rings trouble, the Zabteck housing is really nice, and it comes with sensors, thermostat, hardware, clamps, and a nice bypass hose.