As these units age, this is a common failure point and is a simple end user repair.
Included below are details on the parts including the drawings and specs. Any socket 478 heat sink assembly should work. There are also parts like the Heat sink Retention bracket on Amazon. You can look for someone that sells the Arkua specifically but it is not necessary as long as the heat sink fan assembly is rated for at least 20.2 CFM airflow.
The one that originally shipped in the HPC was a Intel Socket 478 cooling fan assembly made by Arkua, part number 838T-6B (I think that the 838T-7G is the new model). The fan itself is a 60mm x 60mm x 10mm two ball bearing, 12VDC fan. It had a 4,800RPM motor and max airflow of 20.2CFM. See the drawing below.
Shouldn't be too hard to find a replacement. This one looks like it is the fan Arkua used but any the same dimensions and airflow will work.