The two immediately adjacent carbons to the C=N bond can be ruled out due for spatial reasons. Its harder to cram more atoms onto adjacent carbons in a carbon ring.
That leaves three carbons left on the ring. All three should have the same partial charge since the double bonds of the ring will try to compensate for the lack of an electron by forming partial bonds. This suggests that all three are equally possible. Since there are two carbons that can are two away from any attachment point compared to one carbon three away, it is the more likely nucleophillic attack target.
My description above assumes that you are not concerned about the chirality of the final product.