The Isuzu 2.6 4-cyl is the only other engine that will bolt up to your trans. If you use a 2.6, you'll need to take the intake off your 2.3 and bolt it up to the 2.6; it was originally offered as an injected motor. Also, there is no place to bolt up a mechanical fuel pump, so you'll have to install an aftermarket electric unit. Other than that, it should all bolt up the same. A friend of mine has a 2.6 transplanted into his '86, it works fine.
BTW, if you want a good performance mod to the motor while it's out, get a reground cam from
www.deltacam.com
They have a good regrind for both the 2.3 and 2.6, increases power throughout the rpm band. Had one in my 2.3, was real happy with it. It will run you about $60 to have yours done, shipping, and they do have a refundable core charge if you go the exchange route. Good Luck....ed mc.