I did the math and in a perfect world you will need one horsepower for every 62 amps at 12v, or 31 amps at 24v. Keep in mind the belt drive and alternator together may be as low as about 65% efficient. If you're wanting to run close to idle for noise mitigation(a good idea IMO), you might need a bigger engine than you realize. Also, most vehicles' alternators are set to spin three times the engine speed(so typically ~6000rpm). I think this would be a good baseline because too slow and you lose a lot of efficiency--too fast and you will wear out the alternator prematurely. You also may want to look at the work of some of these "green energy" guys. You can gain a lot of efficiency by converting one into a permanent magnet alternator. The catch is it has to spin at the same RPM's all the time--which should be doable with a good governor. The biggest benefit would be it would not need a battery to power the exciter coils in order for it to work. You could even use it to charge a dead battery, or power some equipment all by itself.
crash262: I just put a Honda GC-160 5.0hp on a little longtail mud motor and I can attest it is not extremely quiet. The muffler is a joke. The insides jiggle around so much they literally wore holes through the outside walls. It does not have a camshaft; the "overhead cam" is a plastic--yes, plastic--gear with some funky cam lobes molded into it and it is timed by a rubber belt that runs in oil rather than a chain. I just built it and cannot attest to its longevity, but I can say it is unbelievably lightweight and only uses one-half quart of oil. I'm going to run it hard and when/if it breaks I will switch it out with a 6.5 Predator.
Good Info bud. Yeah the "green energy" or "prepper" guys have some good info on building one of these.