Ok, and this part 91 operations are under the radar of the Easa.
This is the problem...
Will a N plane based in europe be able to operate withh FAA pilots after August ?
No a part 91 operator is not "under the radar" of EASA. They have to comply with all applicable regulations pertaining to FAR part 91 (private flights not for hire) as well as any applicable regulations in whatever foreign country they may be flying in at the moment.
As for your other question. If the operator of the aircraft is not based in EASA land then they can operate the aircraft as they would anywhere else in the world. It does not matter where the owner lives or where the aircraft is stationed at that present time, as long as the operator's base of operations is outside of Europe then they can use FAA pilots. If the operator is a management company based in Europe then it becomes a problem.
g450cpt