Finding a quality battery with higher amperage rating is easy. Any good automotive battery will provide what you want. The challenge is to find a large automotive battery that will fit in the small space provided. I suspect that is impossible.
Honda's goal was a battery that met the electrical load of a standard buggy, is light in weight, and small. Their choice was a compromise.
If greater capacity is required you could always use a boat battery box and secure it in the bed with long cables.
