Would a d16z6 fit a 90 civic?

Yes, it will physically fit pretty easily. However if you are asking if it is a straight swap, no. You will need to convert from OBD-0 to OBD-I and a few other things to make it work and that is more tricky. Try searching "D16Z6 into EF" for more details (EF is the chassis code for your car and is the more common name) ~ZPrime