let me see if I understand the problem:
The pin slips out of the roller and bracket.
The pin can only be inserted one way because the end of it is a smaller diameter then the rest of the body.
Therefore the following should be correct:
The holes in the arm that holds the roller /pin should be two different diameters, requiring that the pin always be inserted in the same direction.
If this is true, then the setup of this collector assembly is similar to what was used on a number of postwar collectors. The smaller diameter end of the pin should be peened over to keep it from sliding out of the hole. If the end is damaged, so there isn't enough left to peen, perhaps one can be ordered, or maybe a postwar one would fit.
Otherwise, I'd go with a screw, as has been suggested already.
The last thing I'd do is replace that whole collector plate.
