The last generator (natural gas burner) I bought for a job was a 13KVA unit that was just under 14HP IIRC. It was for a company project. I recall that there is a cut-off around 13.5 HP or so that requires an EPA permit if it was exceeded.
It may not apply to homeowners, but I would check just be sure your local codes.
I've seen a lot of Generac units out in the fuel plumbed for natural gas and the techs I've spoken to appeared to be satisfied with it. Propane or natural gas, it's an easy conversion with a few small parts. If I were to buy a NG unit, I would get the propane conversion parts just to have handy incase your NG supply was interrupted. The big difference is an orifice and maybe a regulator. All can be changed out in less than a minute if you are handy.
The OP's mention of his neighbor's Chevy 350 running a generator gives me a great idea. I've got a BMW 2.5 liter engine sitting around collecting dust that I could put to good use! How hard could it be to bolt up a generator to the engine if I build a stand for it.....hmmm I engine have a spare 911 engine
(running hard on the track on weekend, the next weekend sitting on a skid cranking electricity to the house) For quiet ...I can weld up a good muffler.