voltage regulator and split charge with second battery

hi guys, is there any sort of external voltage regulator for a micra k10 alternator as im seruiously dropping volts with my sub or is there any larger output alts that are as close to direct fit as possible.
also what split charge would be best if i was to add a second battery in the back, at the moment im just running a 15" sub with 600W amp but will be upgrading to components in custom pods up front off a 2 channel amp and 4x12" JBL subs in the backlist the pic below off a mono bass amp