Welcome to the forum.
You didn't say how interested you are in your gun being "Historically Correct".
That means it would look very close to a rifle that was made during the Flintlock period which pretty much ended around 1830.
I'm going to guess that you are more interested in building a nice rifle that is not very Historically Correct but is well built out of good parts and will be fun to own and to shoot.
Based on this, you have a decision about the barrel to make.
The standard 13/16 inch straight barrels come in both 36" and 42" lengths. If the barrel channel on your stock is made for the 42 inch barrel you can still use a 36 by just shortening the wood.
A 42 inch long barrel is more "correct" for a longrifle but because your probably going to use a straight 13/16 inch barrel "correctness" doesn't really matter.
(Historically, rifles made during the Flintlock era used swamped barrels. That is not to say that when the machine age started and straight barrels became common that no one made a flintlock that used one of them).
You didn't say whether the stock had a mortice cut for the lock.
If it did you are pretty much stuck with the lock it is machined for.
If it was not cut for a lock you have a large choice to choose from.
For a 13/16 inch barrel in a small caliber you could use either a full sized lock like a Siler or you could use one of the "Small Siler" locks.
Both are good German style locks.
You will have to decide whether you want a simple single trigger or a Double Set Trigger. (The kind with two "triggers hanging down".
If you want a Double Set trigger look for the kind that says "Double Lever Double Set".
You don't want a Single Lever Double Set" style which has to be "set" before you can cock the lock.
Only after you've chosen the style of trigger you want can you really look at trigger guards.
The Double Set triggers require a guard that has a long bow that can give both triggers room to work.
You didn't say how wide the butt of the stock is so I'm guessing again.
If it is over 2 inches wide the stock could be used with old or new style butt plates.
The really old ones are wide and slightly curved.
The later style buttplates are thinner and as the years went by they became narrower and more "hooked" or radically bent.
If it were mine I would shoot for a butt plate with a moderate amount of bend.
All this talk and I haven't told you the most important thing you need to do right now (besides dream of how you want your gun to look).
Right now, the most important thing you can do is to go to the Track of the Wolf site and order one of their catalogs.
Not only does it show hundreds of parts you can buy but more importantly the pictures of the locks, the trigger guards, the butt plates, sights, underlugs, sights, ramrod thimbols and many other things are FULL SIZED !
That will allow you to measure the picture and your gunstock to determine what will work and what won't.
Here is a link and at $7 plus postage it is a bargin that every one who is thinking about building a longrifle should buy.
http://www.trackofthewolf.com/cate...&subid=38&styleid=135&partnum=catalog-17&as=1