I need help with an engine swap and the electrical aspects
 

Ok so i recently put a 13B from a 91 Rx-7 into my 88 Rx-7. Now i have run into several problems. I dont know if it would be easier to "change" the wiring harness to fit the engine or if i should change the ECU altogether. If i have to change the ECU, would i have to then change all the wiring behind the dash? Or should i just adapt everything to fit the new engine with the older ECU? Which would be easier? If i end up changing the connections, is there a semi-easy way to do it? (the fuel injector plugs are a problem) If anybody could help out it would be greatly appreciated. Thanks.

you won't be able to change the ECU because the under dash wiring is different, so it won't plug and play.

If it were me, I'd put the S4 crap on the S5 block(and port the intake manifold), and then everything would be normal. or you could modify the wiring harness, but that might be a little tricky.