KLH Model 8 - as per title - isn't a speaker, is it? I guess we're talking about the old Model 6 as per post?
You've done well to keep these going for so long. Aren't they over fifty years old? If you still enjoy them and wish to keep using them then I guess repair is required. From what I read, the crossovers are simple so only contain 2 or 3 capacitors. If you have enough knowledge, I'd try to replace the caps first (best replace all in both speakers), because it will likely be cheap - though finding modern equivalents may be problematic. Have you any test gear you could use to check out the caps first? Even simpler to check out the tweeter to see if it has gone open circuit or shorted out.
I'd be really surprised if you can source an original tweeter, if that turns out to be the problem. In that case, your best bet may be to contact speaker repair specialists such as Wilmslow Audio or Falcon Acoustics - both long established, reputable companies. These companies can often suggest equivalent modern replacement parts. No harm in giving them a call.