I don’t even own one yet but here I go: the little microcosm engines some of us have ordered have been known to roach the ignitions easily. So I would venture a good idea may be the lowest voltage that works. But yours is obviously a different beast, very nice!
Thanks, Tim. I found that these batteries were recommended for some engines
Interstate SEC0905 SLA Rechargable 6V 4.5Ah Emergency Light Battery - This is an AJC Brand Replacement
https://www.amazon.com/dp/B01DOALUUG/ref=cm_sw_r_cp_apa_i_1arOCbMQFHQFB