While I am most intrigued by the low RPM Lister-types, I think building a generator from the Chinese diesels and generator heads would be more practical. We're talking just over $1K for the major engine and diesel components.
I would try to build my own Coleman-style B&S basic genset but after pricing the turn-key diesels in the 8-12Kw range I think it's much easier to justify the project.