PSUs are repairable with very little tools and in home conditions but you need to know what you are doing and good schematic helps.
In this case it's not financially feasible to repair only one PSU of that kind. Those are low tier PSUs and repair may cost more than it's worth specially considering how it broke in first place. There are man safeguards included in computer PSUs but none to deal with hundreds of Amperes a car battery can produce. Considering reversed polarity, damage can be disastrous, someone's lucky only PSU broke and battery didn't explode !! Facefull of sulfuric acid is not a pretty sight.