I was under the impression that when one wants to upgrade his tires/wheels, there are specific rules so that after the upgrade the car still runs as it was intended and the speedometer is still accurate for example.
So, in my experience if one has, say 125x80x12 and wants to upgrade, he would need to go to 145x70x12. Now, if he wanted to fit bigger wheels (provided there are no issues with wheel arches and/or the braking system), the way to go is 155x60x13! Actually I know that from past Alfa Romeos I used to own; when started with original 165x80x14, you had to go to 185x70x14, and then the last Alfa Spiders came with 195x60x15! But in this case, even the base 1.6 Spider had the upgraded brakes of the 2.0. I don't know what the case is with the 126 with regards to 13' wheels, but it's a fact that 145x70x12 works as it should. Maybe someone here knows if it is possible to fit 13' wheels to an air-cooled 126? I also know that this was standard option for the BIS, but I don't really know anything about this version and its differences to the original 126.