I had the misfortune of having an Arona 12 hp in the 70's (Peter Nicholson of Camper & Nicholson and I once cried over each other's shoulder about these engines....). The manufacturer was located in a little town South of Milan. Went there once to get spares, but can't remember the name. Maybe the italian consulate's commercial section could help you.
john
P.S. Forgot to say that as well as the engine we have available to break for spares, we also have some new parts and a set of gaskets which were only made (at great expense) last autumn, plus service manual/handbook etc. I don't know how familiar you are with the engine, but we bought ours from new, and altho' it's a bit 'agricultural', have found it very reliable ( it has done a trans Atlantic 2 year cruise). Apparently Lombardini bought out Arona to stop them marinising their engines,(The bottom half of the engine is a Lombardini) and some spares are available from them.
Get in touch via e mail if you need any further help/info to JeanneLatham@aol.com