Just for info, other than the chinese 311, all AR 20" guns with handguards have a triangular handguard retainer on the front!
You will need to replace the following from front to back:
flash hider,
handguards,
handguard retainer, you will need to go to an early version
pistol grip, (I put A1 pistol grips on all my personal AR's I detest someone trying to tell me where to put my fingers!)
Butt stock, the origional one was about an inch shorter and had a pinned on sling swivel
That stock will come with a butt plate that is quite different that what we are seeing now.
Hope this helps,
Scott