You'd have to find a new old stock uniglide cassette on ebay, or someone here might have one. They're not produced anymore. Thing is, that's just cogs, you don't get new bearings. If the bearings are an issue, its the freehub body you'd want to replace.
Not sure if you can buy a new hyperglide body to put on that hub. Folks here might know, because they deal with older stuff. Shimano is not very helpful letting you know what fits what for upgrading older stuff. The mechanics section would be a good place to put a second post, as well.