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Here's an old M16A1 with a modern day MagPul plastic magazine inserted, these magazines are good, and this rifle as pictured would likely perform flawlessly.
Yet more improvements the M16A1 features over the M16 would be for the outside of the magazine well to be machined in such a way as to provide a raised 'fence' around the magazine release, as it wasn't entirely unknown for someone to accidentally bump it and drop their magazine on the ground, protecting it on the sides like this leaves it just as accessible for the operator as before while minimizing the chances of hitting it accidentally.
Further, the flash hider was replaced, the M16A1 actually did use the three-pronged open flash hider as you saw on the M16 and SP1, but this thing could be inconvenient, as in the Vietnamese jungle it wasn't unknown for the prongs to snag on vegetation and other things, if at enough speed potentially deforming or breaking the prongs off. Eventually they were replaced by a closed front slotted 'birdcage' design, as you still see today.
An interesting note is that the original bolt-carrier group on the M16 was actually chrome plated, presumably to combat corrosion, more is that the A1 does not have a chrome plated BCG, and once the A1 was rolled out, they would actually go back to any M16 currently in the field (as soon as it came back to whatever armory it ended up in) and replace their BCGs with the A1 style, hence you will sometimes also see forward assist serrations on these rifles in spite of them not having a forward assist plunger.
I don't know if M16s in inventory would be given M16A1 recoil buffers, but assuming they would stay in inventory and have changes like the BCG swapped out, presumably they would go through the effort and expense to make the rifle function with issued ammunition, even if these rifles would probably be issued to rear echelon units, sold off to police departments domestically, or given away as foreign military aid.
