s4 and s5 engine parts interchangeable?
 

I have a blown 91 engine that I want to rebuild. I also have an 88 engine and was wondering if any of the parts from the 88 eg. housings etc could be swapped over to the 91? Thanks in advance.

Yes, you can use most of the parts from either engine...but you have to know what you are doing, there are differences with each of them. What are your intentions?

Well I want to replace anythng that is wrong with the 91 motor with the 88 stuff. What are the exact differences between the two?

rotors are different (diff compression ratios)...intermediate housings are different...but not unusable..nothing is really the same cept maybe the e-shaft...but all parts can be interchanged.

Let me put that another way. IF you know what you are doing and what the differences in the parts are, you can swap any of them. IF you don't, you shouldnt make a plan or move until you find out what the effects will be on the rest of the setup.