The capacity of the alternator won't help your starting problems, unless the battery is chronically undercharged for some reason. A 48 amp unit will keep the battery charged OK unless you've got a lot of high-draw accessories like a high-wattage sound system that's running wide open all the time, or a lot of accessory driving lights, etc.
An alternator supplies the current required to run the ignition, air conditioning, fuel injection, fuel pump, etc. before it uses the leftover capacity to recharge the battrery. The only reason to have all that alternator capacity is to feed a lot of greedy accessories. 12 volt generators with a 25 amp output used to keep a battery charged, but there wer not many accessories to feed. It sounds like you've got a sulfated battery, not a lack of alternator capacity.
A battery that sits for long periods of time without being exercised loses a lot of cranking power, and it will only be good for one attempt at starting. Have somebody do a high-load current test on the battery. I'll bet it fails!
Jerry
