claudia:
Cyclic is 100% correct on the modifications. And you are correct on the Type Certificates under the Bi-lateral agreements. The reason for the TCDS required conformity statement by the origin aviation authority is the bi-lateral agreement.
And just as Cyclic said the modification must be removed and the aircraft returned to it's original TC condition or the modification must go through a FAA approval process BEFORE a new airworthiness certificate can be issued to the aircraft.
Not a good place to be if importing an aircraft but it happens more than you think especially on the small plank wing side.
W1