A conventional charger is fine. Actually... I like "Dumb" chargers for the AGM batteries. You can push them, and help keep the sulfate down.
As far as a maintainer... stay away from them. it can boil a battery dry, and kill them prematurely. I recommend just checking the battery every few months over the winter, and charge properly as needed. Heck... my boat sits all winter long, and will get a charge in the spring. No other maintenance when cold. (and my batteries are on their 7th season) For my smaller toys with little AGM batteries... I find they need a charge about every 3 months in the winter. I can get 4 to 6 years on them that way.
Oh... leave the battery in the toy... just disconnect the negative wire to keep parasitic drain down.
