I'd suggest a new freewheel and a new chain to go with it. For 6speed is quite cheap.

Another approach is if you have more freewheels at hand. The cogs on the freewheels are threaded onto the one way mechanism and you can unthread them and mix and match your own cassette - but it's such a hassle that may not worth it, and the threads on the freewheel cogs are not standard.. at 6speed you have lots of brands like Atom, regina, suntour, shimano, etc.

But given a larger stash of freewheels to have all the correct part, mix and match sprockets onto them was common practice in the 80-90's - beware that it's a PITA to remove the cogs from the freewheel, you need a large bench vise and a very large chain whip and a hammer of your choice

http://sheldonbrown.com/freewheels.html