I really like the idea! It would be very fun to play through different levels, constructing various cars or not only cars :) For me it was a bit stressful as I had to think about puzzles, keep track of time and look for opossums that didn't want to show themselves :D I would maybe probably get rid of the timer, as the game can be still fun (or even more fun) without it :)
Regarding the art I would probably take care of the hierarchy of the elements — right now everything lookss very powerful and sometimes it can be hard to focus on the right things. Like in the tutorial, for example: https://gyazo.com/c004d837b075b63346e2a7c95ea4393f all the elements on the screen are bold and big (especially the black frames), and the least noticeable thing is actually the tutorial text, as the font is thinner than anything else :) Maybe it would be better if the text attracted the most attention in this scene!
Your way of thinking and creating challenges is cool and I believe that this game has lots of potential :)