The 88-91 Si ECU is called the PM6.

For the others as to why it is easier to stick with an 88-91 ECU: The wiring harness currently in the car can be adaped to the new motor itself pretty easily. However, the 92-95 ECUs have a completely different plug on them so to plug one into an 88-91 harness requires either a $200 jumper cable or a whole lot of rewiring.