Issues are it is more difficult to make a rimmed semi auto then a rimless semi auto. Not saying its impossible, but virtually no one has done so in at least 50 years. You would basically need a new design from scratch as it would be very difficult to modify a existing 'modern' design.
Its not simply a matter of changing a barrel out, you would require a brand new bolt face (likely a new bolt due to how much larger the bolt face would have to be). A brand new magazine design and the issues that come with that (would have to be heavily tapered, look at a Bren magazine to get a idea). You would also require a adjustable gas system as there is so much variance in 7.62x54r. And on top of that it would have to be reliable to be any interest at all (which is much more difficult than you think).
In regards to the 7.62x54r modern MGs, look at how they are fed, by belts. The rim isn't a issue as it is always held in a separate position from each over round. Magazine fed is much more difficult, with solutions being like the Bren, or like the DP-28.
Too much of a niche market to be worth while for most people. Its one thing to convert a bolt face and rebarrel a rifle to take 7.62x39, its another all together to take a rifle platform and try to adapt it to a rimmed round.