Our facility has buildings up to 50 years old, full of old Continental and Crouse-Hinds equipment. All of the unit substations have been replaced over time, but the original MCC's and panelboards remain. Naturally, this old equipment has no AIC information labeled on the nameplate. Does anyone know what these were rated at, or run across old data sheets that may be of some use?

I haven't seen any but you could take a couple of different approaches:
1. Use the lowest AIC breaker rating in the panel for the panel rating.
2. If those are unavailable, use 10kA as that is the lowest ratings I have found. Some people may advocate the lowest should be 5kA but I have never seen one, although they may exist.

5kA was not un-common up until the mid 70's.

The working life of electrical equipment varies, but much of your equipment could be well past its useful life.
This table is from Siemens. Temperatures are in the centigrade scale. If the equipment is as old as you say, I recommend you write a dated memo to your supervisor recommending a study to see if any of the equipment should be replaced, and keep a copy of the memo in your own files. Your facility sounds like literally it is an accident waiting to happen.

http://w3.usa.siemens.com/us/internet-dms/btlv/PowerDistributionComm/PowerDistribution/docs_MV/TechTopics/ANSI_MV_TechTopics15_EN.pdf

OK, explaining the Arrhenius equation which more or less seems to work for insulation. Now, the example given in the Siemens document is theoretical. What practical values should be used for actual insulation in service?