I have searched a bit but wanted a definitive answer to a few questions,thanks in adv...

I can get a 95 sidekick 4door automatic that needs a engine head for a really good price. I would like to put in the complete drive train into a 98  x90 2wd 5 speed that currently has NO engine or trans at all,it has been completely removed.
I would like to keep the airbags and abs brakes working so im guessing I need a  96-96 automatic ecm but will it work with the auto trans,and if so what mods to the wiring/trans will I need?
What about the engine?
4wd drive switchover?
I think that it's too much trouble,I would rather have a 96-98 5 speed for the switch, but the deal on the 95 is pretty good....

well the deal fell through,thanks for the people who viewed  my question.I am still planning on getting my '98 running and turned into a 4wd  so If I get a good donor car I will post again...

Going from 95 and down to 96 and up can be tricky. 

As for the engine, you will need to swap your distributor, oil pan and oil pump.  I also had to swap the brass water intake tube that goes under the intake but that might have just been a JDM thing.

As for the computer, you can probably get away with a ODBC II ECM, but I have no idea if the x-90 had a different computer then the Sidekick of the same year.  You might need some different sensors as well, a crawl through the FSM's will help you figure that out.

Your center console will need to be swapped out, going to the auto interior instead of the 5 speed.  I would also swap out the pedal cluster to get the wider spacing and removing the clutch.

The 4wd switchover can be done, but your replacing pretty much the entire front end not including the a-arms.  Unless you go straight for a straight axle conversion most claim it is easier to just buy a 4wd to begin with.  Not really an option with the x90 though, they are a bit more rare. 

What sort of wheeling are you planning on doing?

At this point I was just going to "swap over" the parts in order to make the 2wd x90 into a 4wd. Since the deal fell through on the 95 I'm not sure what I'm going to do with the 2wd x90. Right now I have a good running but cosmetically rough '96 x-90 that I can take parts from, or use the '98 for parts. I just hate do do that on such a rare car.

My hope is to find a inexpensive 96-98 trasckick 5 speed and swap it over to the 2wd x90, that should be the easiest to do. Probably keep them stock for awhile then lift the '96 since i wouldn't feel so bad if it got dinged up.

I have thought about the straight axle conversion but that is a bit out of my price range...

When you figure out what your going to need help with, and have the parts to do it. Re post your questions and im sure you will get some good advice. Till then "good shopping". 

Thanks guys for the help, I am going to keep looking for a 96-98 in order to make the conversion somewhat painless.

Keep in mind, an x90 is a 2 door trackick frame and drivetrain, just with a different body.  So if you look at doing drivetrain swaps in the future, any info you can find on 2 door trackicks should pertain to your x90.