It’s a little hard to tell from the photograph but it looks as though the cheeks of the sheave box are bowing out, maybe it’s a loose fit in the slot in the mast?
Have you tried just squeezing the assembly in a vice to push the cheeks back until they are closer to the sheave, then packing the opening in the mast so that they cannot spring out again? Some pieces of aluminium cut to size and epoxied in place should do it.
If not , although you may not be able to get exactly the same fitting, you can get replacement ones from the likes of Pinnell & Bax, don’t be worued about filing out the mast to make a new one fit, the alloy is quite soft. Pop-rivet a new one in or use self-tappers but make sure you use a [assivating gel to prevent corrosion between steel screws and alloy mast.