There are several differences between the two.
Differences in the 89-91 13B motor:
-Engine computer speed was doubled. (Better ECU)
-higher compression rotors.
-Side seals on rotors were reduced from 1.0 mm to 0.7 mm.
-Redline increased to 8000 RPM on the normally-aspirated (NA) engines
-A rotary valve was added between the intake tracks on the NA engines, opening at about 4500 RPM. This valve opens and closes to create a ram air effect.
-The airflow meter was also changed from the flapper-type to a sliding cone.
-metering oil pump was changed from a manual pump to an electronic model
-Engine output of the NA 13B is 160 HP and 140lb-ft torque.
So putting an 86-88 motor in an 89-91 can be done, but you need the 89-91 ECU to control the electronic metering oil pump and you need a lot of little odds and ends. Unless you can get the whole front clip from an 86-88 to pull parts from, it probably will not be worth the hassle. Oh....you will also lose horsepower because the 86-88 motor is good for about 146 HP and 138 lb-ft torque. I used to own a 91 myself and have helped several friends with swaps in the past, so i've learned quite a bit about the 2nd gens.
