I am pretty sure HHKB are all ABS cases. It is unlikely for them to make HHKB to be all PBT (refer to the case with HHKB Pro and HHKB Pro 2 featuring ABS spacebars). However what is known as a fact is that HHKB Pro JP Type-S was marked on PFU website as ABS for the case material.
As for the question the OP has, I am unsure of an affordable fix for the HHKB. If only one could source spare parts for HHKB at a more affordable rate, I guess one can only resort to gluing or otherwise other means of hacks to avert the issue I guess.