if changing to Sdi you'd probably need the ecu, looms and maybe the clocks too. The physical bolting in of it should be a straight swap, but don't quote me on it!

******** VAG Tech Advice *********
If it dont run right cold, change the blue temp sender.
If it aint got a spark, check earths, coil connections.
If it idles like ****, clean the ISV and throttle body.
For all other problems, use a hammer