Obviously a 120aH lead acid battery is the cheapest way to go, but ideally, a DC-DC converter should be used to maintain constant accessory voltage as the pack starts sagging.
I would do a 48V pack & charger (lithium, of course
) with a DC-DC convertor down to 13.5V.
Is it possible for you to chuck the alternator belt, or does it drive other accessories? The nice thing about my Civic is that it has three belts, one for each accessory that I could potentially delete.