I own an SR9 with woodgrain thumbhole stock and forearm. From my reading, I believe it's based on the HK91, but I'm needing confirmation on this. I'd also like to know if I can legally replace the stock with a seperate pistol grip and stock such as Magpul, or HK parts, as they are in poor condition, and I'd like to upgrade. Basically, if parts fit the 91, will they fit the SR9 and are they legal? Thanks for any assistance.