One of the things I have not gotten around to doing, is measuring the efficiency of a 5kw generator head, its brushed, no regulator at all.
I believe they achieve regulation through saturation of the rotor's core.
Anyhow I suspect they are about 75% efficient at full load because it has a 9hp engine, @9x746watts/5000.
I achieved about 75% efficiency from a 2 hp induction motor that I turned into a synchronous generator, generating about a kilowatt worth of power.
far as i can tell you should be able to exceed the nameplate efficiency of an induction motor by converting it to synchronous, but the real gain is from reducing the volts per hz, reducing core loses.
at nameplate volts per hz my 2hp induction motor consumed something like 240 watts from the outlet, running backwards as a synchronous motor at unity power factor.
I suspect a 5KW generator head would be on the order of 1 hp of losses at no load, but i hope its not that much.
if you want to make a 24v generator, i would consider rewinding an ac generator head for 3 phase, 24vac. its only 24 volts so you won't need that many turns per coil which makes it much easier. then rectify to dc, and reduce the rpm of the generator to charge the batteries at something like 2/3rds nominal rpm, 1200 or 2400 rpm.