generally speaking, for slight modifications to tilesets for KS use, as long as you don't claim that you made the tileset in the first place, should be ok.
I don't know what you mean with "might put it in a game", but if you mean any other game than Knytt Stories, that's a no, no matter if you get contact with the original author or not, because I believe the submission agreement requested exclusivity for tilesets. Meaning they cannot be used for anything but Knytt Stories.
As long as you just make a level with a remix of the tileset, and
give proper credit, I'd reckon you don't have too much to worry about.
...yet-oh see mind:
would be his email address, which could be found out sorta via his website that's given in the tileset...if you haven't tried that, do so, if you did, regard the above comment.