It sounds like you're looking for a high powered DC-AC inverter. I've seen these used in small generator projects and most were designed to output the constant 60Hz you'd expect from a real-life mains. I don't know any models by spec. or name, but you can probably check with some generator or solar equipment retailers.
Inside a car engine bay the heat can do horrible things to an alternators efficiency. In an open air enviroment you can expect better results, I guess It would depend on your planned duty cycle and rpm of both the motor and alternator. In my setup for charging 12V batteries and solar backup for a beach shack i will be using a 120A Alternator driven from a 5.5hp Chinese Honda clone motor for 43%.