Easiest is a conversion kit to a normal wheel with a geared hub - Superstar Components do a nice but inexpensive set of spacers. Let me know if you want cogs, I've a few in the toolbox.
Otherwise for high-end stuff have a look at a Phil Wood Kiss-Off hub and White Industries freewheel. I've done thousands of miles on WI freewheels, and the Kiss-Off is as smooth as new after two winters.