Original fitted battery was 28 or 30 Ah, nut now hard to find (or not produced?) 45Ah is the smallest available now, and excellent for the R4.
I buy whatever fits. In the UK the original battery is £35, but the battery supplied for the Austin Mini is the same specification just the terminals are reversed. That costs £20. Easy - turn the battery around and the terminals are at the correct ends.
Just need to be careful to get a battery with the bit at the bottom for the battery clamp.