Outer Limits 4x4 Board

Is there a bigger alternator that will fit in a Vitara, i think standard is 50amps, ive got 2 vits, ones 93 4d and 89 2d project. The 4d is set up with duel batt and redarc smart start. When running aircon and fridge for a while the smart start starts clicking to charge alternate batt, ive got the fridge (cooler not compressor type) on the second batt nothing else except when winching ( which i need more amps for anyway ). should i be running the fridge off the main batt and then switch over to second with a relay when the engines off.
Any ideas appreciated.

i dont know if an alternative is available but is it possible to rewire or rebuild the alt to produce more power/current ??