Well the turret on the 85 is quite different, it would almost be worth carving a new one out of balsa, although you could build on the existing one using modeling putty. Either way, you need to add a cupola from a small disc of plastic on the left side of the roof in the commander’s position as they all had them.
The gun is best made from a thick nylon bristle from a kitchen broom or similar; a quick trip to the Dollar Store or Pound Shop will provide literally hundreds if not thousands of gun barrels, ship masts, flag poles, spears etc for all your table top armies - without triggering the wrath of she-who-must-be-obeyed
The 85 is mounted lower in the turret than the 76 so cut off the original and make a hole in the center for your new gun tube. And don’t forget to post pictures when you’re done!