Hi Andrea,
According to my books the Ardea had two sizes of wheels fitted 145 x 400mm and for export 4.00 x 15 inch. So if you could track down some 15” wheels or have the 400mm wheels modified with 15 inch rims that might solve your tyre problem. Tim Burrett has converted some Aurelia wheels from metric to standard rims for his and others use so that they can fit competition tyres. Some Ardeas were also exported to the USA so it would not surprise me that there maybe stocks of 15 inch Ardea wheels in some forgotten store somewhere.
Chris
that answers a question then, at home i have 1 15inch aprilia wheel, its the 1st series solid type wheel not the ventilated type, so i presume its an early ardea rim, another possibility would be to fit 1st or 2nd series appia wheels, not to different in appearance and 15 inch