There is no single kit to make it look exactly like an SD, however you can get pretty close.
My suggestions would be:
1) Walmartgreeter - was 3D printing very good SD handguards that are perfect direct fit on to the GSG16.
2) GSG5 conversion kit gets you the cocking tube and round front sight and deletes the front half top rail, and gets you the smooth lower receiver / pistol grip. (In Canada only available thru IRUNGUNS I believe with the purchase of the rifle)
3) Airsoft CYMA collapsible stock.
This gets you pretty close and with little effort and not to pricey. It is where I am with mine.
Things that are not done yet but can be with some more effort the rear top rail can be grinded and made too look more like the original and addition of the rear drum sight. I have a small red dot so the rail is ok for me and still looking for a rear drum sight.
The silencer / barrel shroud is much longer than the SD. Dlask is manufacturing short barrels that will need to be installed by a gun smith.
Check out the sticky gsg16 modification thread to get more detailed info.
Hope this helped.