I built up a wheel for my R20 on a Sturmey-Archer 8-speed hub with an integral drum brake. I found one of these hubs for 80 bux on Craig's list, but I had to cold-set the frame to 135 mm. This was a pretty inexpensive way to go, but it might take a while to find a cheap hub. Bike Tools Etc. has SA hubs for twice that. Other versions of this hub are different widths- you will need to get the right width.

You could probably use your existing rim if you're clever about spoke lengths and mathematics- 28 spokes on a 36 hole hub means lacing all but 8 spokes. The spokes are 14 different lengths, and a lot of care needs to be used when lacing, but you'll need to get custom-cut spokes anyway... I did it once with a 40 hole hub in a 36 hole rim.