Not sure on the original battery type. I stuck in an Bosch E23 Varta Blue Dynamic Car Battery (70Ah, CCA 630A) on the basis the c same battery works fine in my RX-8
Having a high CCA seems a good move for a rotary.
It's a bit smaller than the original, judging by the battery tray area, so if you want the battery to fit snugly, you'll need a bigger one
E23 battery is 261 x 175 x 220mm high (inc terminals)
RX-8 230 Bridgeported: Racing Brake Disks & 4 pot front calipers,Full Racing Beat (RB) SS Exhaust system,RB ARB's F & R,RB REVI intake,Tein MonoFlex Coilovers & front strut brace.
RX-7, KIMI 1, 1st Gen S2, Silver,1983 - SORN
RX-7, KIMI 2,^^Ditto - now sold
RX-7, KIMI 3, 1st Gen S3, Red,1984