The exact size is hard to tell without seeing exactly which wheels you are talking about. Most (but not all) X1/9 wheels are 5j13 with the standard Fiat 4x98 PCD with tapered seats. Very early wheels were only 4.5j13 while there are supposedly some examples of 5.5j13s supplied as standard.
Be very careful with earlier wheels as they do not have the bead on the rim necessary for tubeless tyres which makes for a whole load of trouble. This only seems to apply to the 3 spoke boomerang style wheels but they were the most commonly supplied.