the stock size tires are 125/80R13 most likely, my 1981 comuti-car came with it's original 135/80R13 snow tires that I have replaced with new 145/70R13 tires from
Green Autos Home (Tim@greenautos.com sold them to me, they are in Janesville, Ben), but those 145/70R13 tires are a bit wide, they tend to rub while turning sharp, but they grip like mad, I have to be on smooth pavement or gravel to spin the tires, and cornering is not an issue at all, and are much quieter I ever could have dreamed.