Get at least a 65 amp alternator. 100 is better. The alternator will not supply any more current than the battery needs, so ignore those low-amperage suggestions. Even a 12 volt generator had a capacity of 35 to 50 amps, depending on the model and application. I can't imagine any reason you would need an alternator with less capacity than the generator it's replacing!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!