You have a few options on attatching a stock.
Here are essentially your folding options.
Ace Standard Manual folding Mechanism
Ace Push Button folding mechanism
Ace M4 Style folding mechanism
All of these you will have to drill and tap your rear trunnion to attach and that is the way im going w/ my M4 folding mechanism.
Or you could take a stock like the Carolina Raptor stock and attatch right to the rear trunnioin after drilling and tapping or if you wanted a M4 type stock w/o folding mechanism same treatment on the trunnion and attatch M4 CAR BUFFER TUBE PIGNOSE STOCK ADAPTER an