You can probably fit a Shimano Nexus 3, though you may need to respace the frame. The Nexus 3 is readily available with a coaster brake, and has an OLD of 120 mm. I've seen listings for 127 mm OLD hubs as well, but the one available at Amazon and Universal Cycles has an OLD of 120 mm.

I would go with the Nexus 3 over the Nexus 7. While the Nexus 7 does have more gears, with fairly even spacings between gears, the Nexus 3 is smoother operating and more durable.

Of course, you can't go wrong with a Sturmey hub. They are the standard that the others try to live up to. My nexus 3 works great, and I've also had a vintage Sturmey Archer AW hub that worked great.