I rather like the idea of keeping the GPR you have and buying another GPR kit just for the parts.
Just ballparking, the parts for a rifle, bought individually would cost about $370 plus the price of the wood.
If you could get the Lyman Kit for $270 that would pay for $100 of the woods price, so..lets see: $270+$130 (a guess)+$50 says you would have about $450 in it.
I haven't seen anything that says buying a replacement lock would be worth the money, so, if I was doing this, I'd use the Lyman lock.
I think if you contact Dick Greensides at Pecatonica River (815-968-1995), he would sell you a roughed out Hawken Fullstock with the barrel channel and ramrod hole drilled for the price of the wood plus about $48.
You would have to talk to him to explain exactly what you want to do and stress that you don't want the 98% inlet stock he can supply. His 98% stock is cut for a 1 inch barrel and a different lock. If your GPR parts gun has a 15/16 (.937) barrel, that's what you would want him to cut.
I'm sure he cannot inlet the wood for the Lyman lock, so you would have to do that.
You could then finish the barrel tang, lock, trigger, buttplate inletting and fitting and have a good Hawken Full Stock Style Rifle.
Although the breech plug is installed and the sight dovetails are cut on the barrel, if you get the Flintlock, your toughest challenge will be locating the barrel keys in the new stock.
Would it sell for the amount of money it would cost you? Probably not, but unless your doing this just to sell it, that's a moot point.
Would you be proud of your new creation? If you take your time and ask for help here on the Forum when you need it, you bet your bippy you'll be proud of it.
I've come to realize that each of us have different desires, whether it is with your choice of a spouse or your guns if it's what makes you happy, then go for it!