When people say "diecast" they usually mean diecast zinc, but the two metals most commonly cast in a permanent mould (i.e. die) are zinc and aluminium. (For some mysterious reason Americans call zinc "potmetal".) The two metals look rather similar, but act differently. Zinc can't be welded, but can be soldered, with special solder and flux. Aluminium can be welded, with argon-arc equipment (Americans use helium instead of argon), and in a limited fashion can be soldered with a really special solder covered with an even more special flux. I think your roller-ends are probably aluminium, but can't tell from the picture. So, it may be that filling the hole with body-filler is the most practical solution available to you, unless you know someone with the right welding gear.
