All lead acid batteries Flooded/AGM/GEL, ideally want to be truly fully charged held up at high states of charge, and never overdischarged and not exposed to hot temperatures. A long slow parasitic drain to low states of charge, followed by an incomplete recharge, will quickly degrade any battery, no matter its price point.
If you can find a cheapo AGM battery which fits and has acceptable terminals to easily attach to your bike, get yourself some sort of charging source which can bring it upto 14.5 to 14.9v, until amps taper to very low levels or start rising, then disconnect the parasitic loads if the bike is to sit, recharge monthly, at a minimum, and you can likely get good to excellent service from it.