I am willing to bet that you dont need a quad rail as most dont including myself. Id recommend a Troy TRX EXT tube w/ top and bottom rail. The top and bottom rail are more than enough for anything you will need. You'll need a new gas block (lo-pro) and a flip up or fixed style front sight. These rails are the lightest on the market and can be had in 7,9,11,13" lenghts. Ive purchased 3 and have been happy each time. They are fairly inexpensive and free float as well.
Id also purchase a new stripped upper