Today, I tried to mount a 5mm ball bearing into the motor shield of the old BR 81 with disc collector:
There is enough thickness and material, so it is no risk to drill a 5mm hole instead of the 2mm drilling. I started with a 2mm-4mm drill and finaly I tokk a 5mm drill.
The motor sound is less noisy then before, but I think it would be better to do the same on the gearbox, too. Alternatively, You can change to the 5-Pole HLA.