You can, indeed, remove the generator end without pulling the whole unit. If you can diagnose the problem as the rotor, that is very simple to pull, but if you can't pin it down the whole generator end is not much more work to remove. You'll want to separate the rotor and stator so you're not schlepping too much weight. No need to pull the diesel motor end.
I got my rotor rewound (a 6kw Northern Lights - similar to your Onan) in Venezuela for under $200 (two hundred) US. Try an industrial generator shop that doesn't have "marine" or "Onan" in it's name. It's just a bunch of generic wire and any competent shop can do the work. An entirely new generator end should be about $2,000 so keep shopping around as 2000 euros seems very high for a rewind. (Unless most of it is for labor to pull the generator, which you can easily do yourself and just give them the rotor. Send me 10% for all the money you'll save ;-)
Good luck with it,
Craig Briggs (the incurable DIY guy), SN68 Sangaris
PS. I'd look around Palermo - Trapani is pretty thin on industrial resources.