I have a similiar bike that i'm fixing up to use as a commuting bike. I thought that since it has a 6 speed rear that it was a freewheel, but now I find out that it is a casssette.
I've noticed that some people are recommending getting a new freewheel while in your original post you state it is a cassette. If it is a cassette, I think it's going to be harder to get a replacement.
By the way, my bike has a 13-30 cassette.
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