If the creator has been inactive for years I think it would be okay to port it over to koth, I would definitely try contacting them first if they have been active in the last year though. People have made payload versions on control point maps after all (Like cp_dustbowl which is owned by valve).
From what I heard way back, you can't get sued just for editing/fixing a map in hammer because of the license or something. I could have been told a load of bollocks though but this seems a common view.
Basically as long as you are not being a dick about it then you should be fine.. (Taking full credit for it, selling it, etc)
No, you cannot get into legal trouble for distributing online work, unless it detracts from the original owners income as it is paid content, or you attempt to sell it yourself. The owner can ask you to take it down, which you should respectfully do, and they can report you to a site administrator or similar, but that's it.