Polaks made a 7.62x51 version of the PKM, but their objective was to feed from NATO-standard disintegrating links. For that reason, the feed system was different (and the barrel was shorter, receiver longer, and the gun became heavier). But the image of the feed system shows traditional Russian feed box, as well as Russian belt and, most importantly, cartridges. In addition, you're in your own world, and don't need to comply with external standards. Just feed 7.62x54 Russian like Kalashnikov intended, jeez. Finally, if you want sustained fire and 7.62 NATO ammunition, why not just use MG3 or FN MAG? The main advantage of PKM is that it's the lightest of them all, but switching to 7.62 NATO negates that advantage. This what happens when people are theoretical otaku in a subject.