It is best to replace the battery with the same, its capacity is 55Ach. Its code is 55D23L. It's also a 75-hour battery, its code is 75D23L.
However, it should be remembered that the increased capacity of the battery can increase the load on the generator, and the battery itself will often be under-charged, leading to a reduction in the life of the battery.
You can also purchase a third party battery. For example, VARTA Asia Dynamic 60 H.