http://www.ecanfix.com/~mdhamilton/ecuconversion.html
Any 1G Turbo EPROM ECU can be made to work in your car but the '90 ones will be easiest.
I'm assuming you will get a new chip or be running 1G DSMlink.
The SAFC RPM input doesn't seem to be effected by the differences in the tach signal between '90 and '90+ cars, so if your willing to live with a funky tach you can just swap the two ECU pins to run a later ECU. (Don't cut the wires! The pins pop right out of the connector with a jewelers screwdriver to lift the retainer)
If you need
ECU part numbers to figure out what's from what.
Steve