R179 (50R) has been replaced and I had previously removed R134 (510R) for testing so I replaced that as well.
After several weeks of waiting for components I received replacements for U81, 2dB chip attenuator and U50, GaAs SPDT switch. I took them to a local electronics repair shop but unfortunately they could not remove either U81 or U50 with their hot air gun rework station, probably due to the large ground plane. I'm not sure what to do now as using a Dremel tool or similar to remove U81 and U50 may damage the board beyond repair.
I think I may reassemble the unit and continue to use the VNA part as that was working OK.