The usual method for alternator control is modulating the excitation winding. Apparently it's not possible here, e.g. PMAC machine.
Small size means no filtered DC input to the down converter. I further presume that you have a battery, so no output filter is needed.
If you want to optimize the generator efficiency, buck PFC could be a solution. Otherwise phase angle control like implemented in the original design s still an option. Power dissipation could be reduced by using an active MOSFET bridge rectifier and MOSFET switch.