No, you cannot modify the existing one, you will need a replacement.
The ECU is completely different, and the electronic package under the hood is different. The 7M-GE wiring harness will not deal with the AFM, CPS, igniter, or coil packs. It probably wouldn't quite deal with the TPS, or ISC.
Basically, it's a bad idea. You need a compatable wiring harness and ECU from a similar year. You can group 1987 and 1988 together, and 1990-1992 together, with '89 being it's own special case. 1989 will work with a 1987-1988 wiring harness and ECU with some minor caveats. 1991 and 1992 will have differences with some connectors from 1990 as well (I know the knock sensors are different, for example.)
