Radiator plastic failure usually occur at the top due to the repeat wet and dry cycle compared to the bottom where it's constantly submerged. Before it fail, it start to turn color to slight brownish from black.
Cooling component plastic once aged, will turn brittle, patching with glue is just a temporary measure because other surfaces will fail not long after that.
Changing of the cover is another option but we have left and right cover. Once your radiator is out of the car, it's advisable to replace both end cover and it may cost you 80% price to a new radiator if not more expensive. Furthermore, the radiator repair takes time, need to park car at workshop for at least half a day. I replaced mine for RM600. Brand new, kao time. 20 min job.. 2 x 1.5L BMW coolant already cost RM60 lor. Why waste nice coolant in a repaired radiator knowing it will fail again..