Existing sprites are not so bad actually. Moto is a concept after all, and also it's more like a sporting thing, so this spartan design fits well.

Also, the game will not look much better if you just change sprites. You would have to add moving clouds, grass, animation, lighting, special effects and so on. It's complete rewriting of visualization code.

If you want to spend $360 on moto, I think it would be more efficient to spend it on advertising like banners and articles or give it to bittrex if they agree to add moto and promote it.