Well, the .308 and 7.62x54 are way too big for the SKS, unfortunately...
However, the easiest conversion would be 5.45x39mm, as AntiBubba mentioned. New barrel, probably some tweaking of the gas system, but should feed pretty much the same as the 7.62Comm, right?
Problem is, 5.45x39mm hasn't exactly taken America by storm, as cool as the round sounds.
Also the 5.45x39mm would negate most of the hunting usages of the SKS, as it's most likely illegal to use .21 caliber rifles on pig and deer in much of the U.S., and presumably even a 5.45mm SKS wouldn't quite be up to long-range varminting.
So, a 5.45mm SKS would do no good for hunters, not appeal to the average Bubba who just wants to shoot cans for cheap, etc. So, it would basically be a delight for us gun geeks and nobody else.
Now, if the Serbs want to start making and exporting SKS Cowboy Companion/Paratrooper variants in 5.45mm, I'd buy one for $150, but that's about it. Fun to think about though.