If the sidekick sport is a 98, then that is the wrong ECM for it (from what I have read). I am not familiar enough with the sport ECMs to know what the difference is between the 96-97 ECMs and the 98s and how running a 96-97 ECM in a 98 affects things. But from what I have read, the 98s used a different ECM than the earlier ones. So I don't know if an earlier ECM will even run a 98 (apparently not, from your experience). I would almost bet that both ECMs you have are good, and since they are not the correct ECM for a 98, that is where the problem lies.
The PO is full of crap. #1....4 speed automatics (which is what a sidekick sport would have), don't have a shift modulator. So there is no way to replace something that isn't even part of that tranny. Now maybe he is calling the part by the wrong name, or maybe you are not remembering the correct name. But if he is right and you are right, he is lying.
Here is my guess on what happened. The person you bought it off of never had it running (or if they did have it running, they replaced the ECM with one that is not compatible...which is apparent by the part number of the ECM that you say it had in it). So #1, either the old ECM wasn't bad and the problem is something other than the ECM, or #2, the old ECM was bad and they replaced it with one that isn't compatible, so that is why it won't run now.
Do you have a way to contact the PO, to see if he still has the old ECM around? I would like to know what the number is on the original ECM.
One other thing to consider....it sounds like the engine was replaced with a JDM (japanese domestic market) engine. And from what I have read about JDM engines, is that some of the sensors are different and not compatible with USA wiring. So people have had to swap the sensors from the old engine, to the new JDM engine. So I would try to find out how much of the engine is JDM and how much is original USA. If the sensors are still the original US sensors, then that is good....at least you can rule out that it is an incompatible sensor causing the issue. That's not to say one of the sensors isn't bad though.
Yeah, it is gonna be hard to pull codes, since the battery has been unhooked and the engine can't be started. There are a few codes that can show up, after the engine has been turned over for several seconds, even if the engine won't start. The procedure is to turn the engine over for like 30 seconds, then leave the key in the run position and then check for codes. From what I remember, it is ignition codes that will show up, so that may point you in the direction of where to look.