The enforcement is done through the competent authority. Now how that works for a dodgy N reg in Europe I am not sure...
I anticipate that the FAA will be really hacked off at getting dragged into an investigation where the relevant overseas authority has let things get to this stage. Can you imagine the FAA tolerating for a moment flag-of-convenience aircraft operating inside the USA against their regulations.
Clearly this accident throws up all sorts of questions not least who engaged the pilot and what was known about his qualifications.
It's a grey area because normally the sanction against a bad operator is to cancel their AOC. The regulations don't cater nearly so well for someone who never had an AOC in the first place.