It's still common courtesy. A while back I made a updated version of the Ecto Goggle label which had used a vector file derived from his own. Even though I'd replaced the graphic with one I'd created, I still felt obliged to post it online for free, and to credit him.
Now I'm not expecting you to give the stickers away, but I do expect Ken to be given the kudos he earned by being the artist who created those sticker sheets. Minute alterations you may have done aside, you are still obligated to ask his permission and to credit him.
Such a credit could easily be in the form of adding a “Modified from original files created by Ken Heugel” to the base of the sheet, or in one of the spaces between the labels.
Please seek him out, it will be worth the effort.