Hello Vito and others,
If you are planning on racing in FIA sanctioned events, get a copy of the homologation papers to see what sizes of rims were homologated for the various models of Abarths. Generally you will have to use Dunlop tires (bias ply) for FIA competition.
Other race organizations are less strict. The Abarth Coppa Mille will let you use any DOT approved tire or race slick.
It has been my observation that many of the 850TC/1000TC/TCR cars, be they originals or replicas, are OVER-TIRED. What I mean is that cars running smaller diameter, narrower tires tend to go faster than the ones with huge rubber. A "popular" combination is 185/60x13 in the front and 205/60X13 in the rear. Looks really macho !! The truth is that at speed none of these motors, with the exception MAYBE of a full race TCR, has the power to break traction with 205/60x13 tires !! Perhaps it would be nice to have the big rubber in the corners (up for debate) but on the straights the larger rubber is just plain slower.
The faster cars all use 185/60x13 (or smaller) all around. They corner just as hard and they have less rolling resistance on the straights. Not only that, they weigh less, so there is less mass for the engine to accelerate. (Works the same way as using a light aluminium flywheel as compared to a heavier steel one).
That being said, I can see little use for wheels any wider than 6 inches.