New Owner - Battery Question

[gmansbu]

So, I just picked up my H2 a few days ago and I couldn't be happier.

The previous owner had an aftermarket stereo system in the car. Before I took posession of the car the guy removed all his stereo stuff and put the factory system back in the car. He did however leave the battery he purchased. He mentioned that I need to get a smaller bolt to bolt down the terminal than the one that is currently in the battery. It can't tighten down enough and the terminal could possibly come loose. I'm bringing the car in this week to get a shorter bolt put in. My questions is do I want/need the battery he put in there or should I remove what he put in and bring it back to stock? Pics below & thanks in advance.