For MGA owners, the most important point of this article may be that we find a 12 volt battery which will fit nicely into the original MGA battery tray with no modification required. It drops right in. The picture below shows two 12 volt Odyssey batteries nested nicely in the original 6 volt battery trays. You must not connect two 12 volt batteries in series for 24 volts. Also consider general advice warning against connecting batteries in parallel (see next page).
One of these batteries will do the job for an MGA (marginally). Cold cranking current capability is a modest 326-CCA. This is similar to the original twin 6-volt batteries, which were not very good in cold weather. Total electrical storage capacity is moderate at 27 AH (10-hr rating), which is less than half of the storage capacity of the original twin 6's. This is why one person was installing two batteries in parallel. The original twin 6-volt batteries had storage capacity of 58 AH. So if you install one Odyssey 12-volt battery, don't expect real strong cranking ability in cold weather. If you leave the headlights on when parked it will be dead in an hour or so. If the generator fails when driving with lights on you will have a similar short period of time to get off the road before the car is dead.
The battery shown above is the Odyssey PC925T, with the "T" being for the SAE type terminal posts. This is smaller in height than you may be used to seeing in a car battery being less than 5 inches tall (not counting the terminal posts). These are zero maintenance sealed dry cell batteries which will not leak if punctured. They can be mounted in any orientation and have very long life and excellent vibration resistance.