Well........
There's no "changing the lock back to flint", that lock is and has always been a percussion lock. So to make your gun a flint gun you'll need a whole new lock and it will have to be fitted to the stock, the inletting for the current lock is very different as the plate is not really similar to any flintlock's plate.
I would just leave it as it is and make all attempts to get it shooting as is if it is certain that it is in safe condition for firing. You have a gun that appears to have been made ca.1850 - 1870. The stock and lock are from that period, they just used a few older parts and made a gun that is perfectly "historically correct" for the period of it's origin.
Don't take anything I have said as discouragement, sometimes I sound harsher than I mean to. Often in fact. Have a qualified and reliable black powder gunsmith look at the gun and tell you if it is safe and tell you what needs to be done to get it operating safely. The, if you feel qualified to do the work, do it and see what it can do. If he says it is not safe to fire, it will look great on the wall over the fireplace or you can sell it to someone that wants it for that and then take that money and start again.