I agree that you purchased a compliant state model, based on the pinned stock and "bull barrel"... I am assuming that you do not have a flash hider. I wouldn't call the barrel profile of the Sport a bull barrel, but it does not have the cut outs for the grenade launcher that the gov. profile barrel has. I doubt that this will cause you any issues.

As far as replacing the stock, all you really need is a new receiver extension tube and a new stock... Buy a "Mil Spec" receiver extension from BCM, Midway, or Brownell's and the stock you like. You can still use the same buffer and spring that you currently have.

You can start monkeying around with different buffer weights and springs, but the reliability can suffer, depending on what ammunition you shoot. If you shoot NATO spec ammo only, you can use the H buffer without issue, and probably the H1 if you wanted. If you shoot cheap .223 or Tula, you may be ok with the H. You can always get the weights and take a day at the range to see what works... go with the heaviest weight that cycles reliably with the ammo of your choice.