I have an even better idea. Keep the existing 400 Hertz alternator. Use the output to drive a transformer that supplies a 12 pulse rectifier that charges a 48 volt forklift truck battery. The best configuration would be to use 2 6-pulse bridges in series to produce the 65 volts
DC that you need for charging which eliminates and ineterphase reactor and current balancing. Since this is the only load you could rig the voltage regulator of the 400 Hertz generator to produce the proper DC voltage.
There is also a solar photovoltaic charge regulator on the marker from
that can regulate say 80 volts DC down to what is needed to charge a 48 volt battery properly using a 3 stage charging curve. You can equip the battery with a 500 amp shunt that allows the charge controller to both supply load and maintain the right amount of charging current.
You could also belt drive a conventional 10 KW 60 Hertz alternator to the front of the engine. You should be able to get a power takeoff shaft that bolts to the front end of the crankshaft where the harmonic balancer is. You would put your belt sheave onto that. Same power takeoff that is used to drive hydraulic pumps, air compressors, and so forth. Take a peek inside the engine compartment of a ready mix concrete truck to see what I mean. The best belt for this application most likely would be a Goodyear Gripbelt which uses herringbone gears for the sheaves and produces less noise than a conventional toothed timing belt.
What you would do with the whole shebang is to attach an inverter to the 48 volt battery to supply 120/240 volts without the generator when the load is light. When the load is heavy or the battery runs down you crank up the generator. Just make sure that you get an inverter that is rated for solar photovoltaic use so that you can tie in some solar cells.
You can also go over to Bodine Electric and get what are called Transbal electronic ballasts that can run directly off of your 48 volt battery. These are ballasts for places where normal power comes off of a 12, 24, 48, or 120 volt battery. They also have central battery emergency ballasts that are for emergency lighting in telephone central offices.