ehime

iTrader: (18)

I just got a smoking deal on a rebuilt S4 engine that is just to good
to pass up. I would like to know though if there is anything I will need
to change on the engine before I drop it in. I have a 89 GXL S5, and
since this is an earlier S4 engine I'm not sure if it is a direct swap or
if I'll need to change anything on this engine to get it to play nice with
my S5.

freemanrx7

https://www.rx7club.com/2nd-generation-specific-1986-1992-17/%2Aplease-help-%2A-s4-turbo-s5-turbo-swap-748406/

stylEmon

iTrader: (2)

there are a few things you'll need to change.
the OMP and front cover are the first that come to mind. searching should help you out, I know this has come up more than once.

farberio

iTrader: (7)

Run the S4 block and S5 accessories.

kungfuroy

iTrader: (6)

center plate on the s4 block has the EGR on it, this needs to be removed/blocked off