The AGM equivalent of the YTZ10S will be fine, just a little more expensive. Much less prone to vibration issue, but its biggest advantage is its lower discharge rate when idle for long periods.
The AGM type motorcycle battery needs a regulated voltage between 14 and 14.8 volts when charging on the bike and the FZ8 is well within this range
Be sure to use the correct type of off the bike charger to charge AGM batteries if you have to charge it at any stage.