S4 vs. S5 FC RX-7 questions

I've got an '86 RX-7 that is my disposable hill climb car, I've done some suspension work to it, has a roll bar, put in a TII rear diff from my parts car, problem is the 2nd rotor lost compression. Not sure if it got too lean when it developed a fuel leak at the pulse dampener thingamabob or what happened :( Found an S5 motor, but in my brief research, there appears to be disagreement as to putting an S5 engine into an S4 car. I know the front cover is different, but could I swap the front cover on the S5 engine for the S4 cover, simply change all of the externals & use the S4 ancillaries & hook it up to the S4 harness & make it work? I don't want to dump the time to do a full swap to piston power, that's what the other car is going through now, I just want to swap rotary for rotary, bolt it up & run it some more. Any help?

Do your research on the harness changes. I know doing the reverse and going turbo (S5 NA to a S4 TII) requires a lot of harness manipulation. There are alot of guys that have done what you are attempting or may attempt and I believe it is fairly well documented at the rx7club. From what I have read doing an NA - NA swap is pretty straight forward, but the harness will be item to really pay attention to.

you can use JUST the core engine technically speaking without too much fiddling, but would have to ensure you run the S4 Metering oil pump as well as a few other items.

Just swapped S4 front cover stuff onto my S5 motor, it's not hard. Like gravityfed says, use the S5 'core' engine with S4 externals and you'll be all right. The S5 has higher compression than the S4 so you'll pick up a few horsepuppies as well.

Be aware of the thrust bearing at the front of the motor, since you'll have to pull the 'snout' off the eccentric shaft there is a chance the needle bearing or its race could fall between the counterweight and the front 'iron', this would immediately lunch the motor on startup! Basically, don't do anything that will force the eccentric shaft toward the rear of the motor when the pulley 'snout' is off the shaft.

Unless you get a turbo trans you will need a kuztom drive shaft. I'm assuming the engine is NA, since there was no 86 turbo, and you didn't mention otherwise. The easiest was to put a S5 engine into an S4 is to take everything off the S4 and put it on the S5 (front cover, intake, injectors, bac, tb, etc). I would recommend going through the extra trouble to use the S5 intake manifolds (either turbo or na). The S5 NA manifolds will make an additional 15 hp over the s4 by use of the variable intake. I'm assuming you are stripping all the emissions junk off it, otherwise you will still need to make a blockoff for the egr valve (the ports on the LIM are different). Check out Rotary Resurection for a fairly detailed swap guide.