Stipe, if you didn't know, this isn't 1991, war is over. If Serbia or Yugoslavia if you more like didn't use MiG-29 in war period I can't see why they would use it now, also I really can't see why Serbia would attack Croatia today or Croatia Serbia. That war era came to an end, I mean about war between your two unfortunately brother nations.
Reason why Croatia need to buy a new plane is not to pay Italy or Hungary to guard you air space, there isn't other reason...