Can’t say whether you can easily fit a higher output alternator unless you tell us which one is fitted currently. What year and model is your car? You don’t drive around with all your lights on all the time do you?
not driving around with them on all tokme but i live out in country side so have main beam on alot and you can see the light getting dimmer as it drains the barrary!! the system i got planned going to drain bigtime on the barray even with power caps!!
I’ve know of people fitting alternator’s from Rover 800’s, and a mk3 transit battery just fits in the battery box. Make sure it’s all got suitable wiring and fuses.
Battery is expensive so mayB try the scrappy and look for a good one, check with multimeter or battery charger/checker…