Dazz, I believe that almost any battery will be better than the lead acid battery that comes with your bike. You will be limited by the space available for a replacement. As a start, take your battery out and measure it. That's the maximum size that will fit. Then shop for something that is no larger. I'd recommend a sealed battery. Glass mat is one choice. The Shori (sp) is much smaller but will give the same energy but a higher cost. It will also hold a charge for a very long time, maybe as long as a year. If you store your bike for a long time that might be important. I went with a gel cell because I can keep it on a charger when not in use.
Growing old ain't for sissies!