Simple Gas Generator question
It doesn't have to be a switcher. With 120V AC you could use an AC transformer to get 12V AC at (somewhat less than) 58 amps, then a rectifier, reservoir capacitor and (if necessary for your application) voltage regulator to get 12V DC. (I'm simplifying a bit: unregulated, 12V AC would give closer to 15V DC)

A PC power supply will be easier to find at 20A+ ratings, but you may find e.g. a car battery charger that would do the job.

My main worry would be that the AC from a small generator might be too poorly conditioned/regulated for a PC power supply and cause it to fail. I might be overcautious here, but the transformer/rectifier solution is likely to be more robust. You don't say what the load is; knowing that may help improve the answers.