Hey there chris, I've been thinking for a long time of doing the same thing, I think for the stock to get a closer look to the m91 it might be better to modify an svd stock (I'll probably use a lam airsoft one since I'm cheap lol) for a more authentic rear sight you might want to use an m72 rsb as they are exactly like an m76 for a gas tube you might not be able to find an m76 tube but a psl tube could probably be modified to work well, I could take measurments of my m76 tube if needed.. An m76 gb isn't easy to find if you can't find one and you have a friend with a mill then I could lend you my spare to copy. Same thing with the combined fs/fh the m76 has, hard to find I would use an m70 or an ak47 pattern (depends if you want to have the night sight or not) I would at least perfect the reshaping of the rear of the reciever with the apex pieces if I were you (I have a couple apex cut recievers to play with myself) for a barrel I might try starting with a mosin barrel if they have enough meat but I'm cheap like I said earlier, how much would a walther blank cost? I wouldn't mind going that route too but my wallet would probably cry lol. For the scope and mount I would go with a zrak on-76 from apex and either use the 4x scope or just use the mount with a larger scope but at least you'd have the authentic looking mount. Anyway those are just some if my ideas I had rolling around in my head what are some of your thoughts?
Oh yeah for mags I would do something cool for mine as in neither of your ideas lol I was looking at modifying svd mags to give it a fun flair (maybe have some psl mags for extra mags)