The game and all it's contents are property of the Pixel Ferrets company and therefore you'll need official word from the devs on this.
As for re-using sprites as a topic, it's not something that's done - at all really. Not only do the graphics of a game distinguish it from any other game, they also create the majority of it's feel and experience. You can't really get away with using the sprites from another game and selling yours as something different, without making a very similar experience.
It doesn't hurt to give drawing a try right? Even if it's really basic, it's yours. It's your property and it's your game. No licensing issues to get tangled up in. I'm a fairly competent artist when it comes to drawing my hand, but I'm challenging myself to draw with letters for my game, which is completely different. Just, give it a go. You might be surprised with what comes out of it