The "rule book" (standards and regulations) is there to protect everyone, including the patients and those supplying and maintaining medical devices. If someone decides they don't want to buy OEM parts, then they are accepting responsibility to meet the required standards and regulations. The OEM would no longer be responsible for failures of the device, as that has would have fallen to the person (or organisation) making the modification.

So feel free to fit non-OEM parts but understand your liabilities!