I'm surprised no one has mentioned Lister-style generators. A very old, reliable design. See
the Cool Tools write-up for additional information. There are one and multi-cylinder versions that produce from 5kW and 20kW, with your choice of single and three phase generation; most turn around 800 RPM and cost less than $2,000. Some even offer electrical welder options! All are quiet and fuel efficient, and most are multi-fuel (wvo).
The largest drawback is that they aren't portalbe and they're quite heavy--larger models are over 2,000 pounds.