I see the surface area problem ... but I don't think the track has to be that long, just enough to move over the snow, right? I think a regular wheel would work for that. Just replace the tire tube w/ some sort of track-like thread that would rotate. You'd get the same surface area of a tire, but the "track" should move over snow just fine.
Keep in mind that I think the SnoRunner was designed for off-road snow use. I'm not thinking of an off-road Snow-ped, only one that can ride on regular streets before they are fully plowed. So small track area should work fine on thin but densely packed snow. After all, my regular tires did fine the few times I rode in lots of snow.
This way we could still use a regular, fully working moped. It would simpy require replacing the rear wheel w/ the one above and the front wheel w/ a ski. Of course, I'd still recommend putting little skis on the sides of the rear for balance.
Just some ideas.