It would probably be less work to pull the engine back out and install a flywheel with holes around the rim and a crank sensor, and use the 90 distributor. Keep in mind that there are at least two patterns of trigger holes on these, and if you get the wrong one, it won't work. You will also need a new clutch, as they are different for the (early) flat flywheel and the (late) stepped one with the holes in the rim. Taking the dash apart is no fun at all, and while the 85 harness will obviously work with the 85 engine, the other ends of it may not connect directly to the rest of the 90 car. Also, 85 is in the year range that is prone to deteriorating wiring, so unless it has already been replaced, that harness is suspect. The bell housing is the easy part---there is a thin area that can easily be cut away where the crank sensor mounts. A bigger question is whether or not there is a place to mount it on the back of the block. There might not be on an 85. You can't just plug the 85 ECU into the 90 harness. Several of the pins are connected differently, and some of the wires don't even exist (to the Hall sensor, for example)

Sorry for what is probably bad news. I converted my 88 from AW70 to M46, and ran into some of the same issues, as the 88 was Hall effect and the 90 the trans came from was crank fired. I had to track down a flat flywheel (and buy a new clutch) and a bell housing without the notch in it (or I could have made a cover plate for the hole) You are going the other way, which seems to me to be more complicated. Good luck with it.