Hmmm it's not as straight forward as you might think, but it depends how compotent you are mechanically.
Off the top of my head you will need the following items off a 4x4
(Anyone feel free to add if i forget something)

Front cross member
Stering rack
Front hubs
Front diff
Front driveshafts
4x4 sump
4x4 pickup pipe
4x4 gearbox with front prop
Modified cradle to mount the 4x4 box
Exhaust will need changed (also depends if your running 2WD or 4x4 exh. manifold)
You'll need to modify the front chassis rails to accomodate the front driveshafts

Also i was reading the other day that the engine 205 block isn't reccomended for 4x4 cars as the design of the block wasn't intended for mounting a front diff onto and could therefore put added stress on the blobk, how true or big effect this may be i don't know.

I've probably forgotten a few things though.