If the races are badly pitted, you'll need a new hub.
I'd agree with SCH and upgrade to a cassette & Free Hub. Easier to service and a wider range of cassettes are available than Free Wheels.
You could replace the hub using your old rim, but by the time you pay for labor & new spokes, you may as well buy a pre-built wheel.
Another option is to look for a donor wheel of a Craigslist or similar bike. That's what I did when I had a R wheel stolen. I picked up a bike with a Free Hub for $20!