Here is how I used to fix this: I take the arm and hold it with pliers about 4-6" over a flame, a gas stove works well. You just want the plastic to get soft, not catch fire or start dripping. Have a cutting board ready. When you see the plastic is shiny and soft, specifically the 6 tabs that engage on the pump, slap it down on the cutting board to flatten and spread the tabs a bit. It should now lock in very tight.
Now, I used to do it that way because these pumps had a lot of life left and it made sense to not glue stuff up for future repairability. These pumps are all nearing the end of the life you can expect from them and I would probably keep it simple and just use a bit of epoxy as once the motor is dead, it is a lost cause so there is little sense in worrying over being able to salvage some small parts.