If it's one of the twenties with 406 size wheels, then pretty much any old-school BMX rims would work. I rebuilt my wheels with Alex DM24's for three reasons (also Sun CR-18's would work).
1) They are cheap.
2) They have eyelets.
3) They are double-walled for strength.
This is a good opportunity to learn wheelbuilding; you want to reuse your hubs. I doubt anybody has pre-built wheels for an R20- I certainly couldn't find any. Modern BMX wheels have axles that are too thick, and finding a 3-speed hub version would be needle-in-haystack.