You can use the one for the current BT99. It becomes a question of what you want to do with the long tang though. Since I didn't want to cut the tang off in case I ever sold the gun with the original stock I did a bit of reshaping of the tang and then used black electrical tape to hold it to the grip. It looks a bit bubba but most people don't far enough past all of the metal to notice the tape.
I installed a PFS stock on my BT-99 Plus. I basicly bought another tang from Browning($29) and cut it long so that the tang tucks up into the gap were the stock slides on. If you bend the tang up it will put pressure on the tang and it won't move around. Mine took a little while to fit but it looks great and I also have the original tang still on the wooden stock should I ever decide to make it original again