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Sounds like you are off to the scrapyard shortly!!!! The Thermostatic Air Cleaner is a much misunderstood and underrated part of the AMC258, it handles cold air, hot air, water and dust which off road is a greater hazard than for road vehicles.
The Carter BBD was popular with most manufacturers in the day so you may be able to find a TAC that fits from another make. In fact any TAC can be made to fit with an aluminium plate, a drill and a jigsaw, just cut out the TAC until it fits over the BBD and then fabricate an aluminium plate to fasten / glue / seal in the bottom of the TAC over the carb.
Alternatively you could buy one of those chrome pancake filters and cut out the right shape in the bottom and drill out some screw holes. However it will never work as well as it will be prone to water kicking up in the engine bay whilst wading and will not duct in hot or cold air as needed by the engine.
As to size of filter, the bigger the better. You will want to take your Jeep to muddy, dirty, sandy places and a larger filter will last better. I know that K&N etc are rated but an ordinary, large and cheap paper one is just as good.