Charlieman said:

Don't give up with old batteries. The Portable uses a lead acid battery, which can be exercised, possibly to rejuvenation. Boaters know people who have battery exercisers which can fix a battery. If sulphination kicks in, it is not necessarily a death sentence.

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It's dead, Jim. It's a 25+ year old lead acid battery. We have a hard enough time finding good batteries from the 90's and early 00's. Replacements are less of a hassle and risk of dramatic failures.

Personally I'd use any combination of 2v cells or one 6v cell that would fit in the old enclosure.