I liked the original, but I made a few changes before printing mine.
-Solid body
-Flat bottom
-Pin-based axles
(These changes were to reduce overhangs, allow for support-free printing)
-Scaled down 50%, but kept balloon inlet/outlet closer to original size
-Optional spare tire

When I did finally print it, I ran into two problems.
1. It doesn't roll. Whittling down the axles and opening up the hubs with a drill bit got me wheels that freewheel very nicely in the air, but when I put the car down and push it along they just gain a bunch of camber and slide along, maybe 1 or 2 wheels spinning at a time.
2. We haven't had a kid's birthday here in quite a while, so the balloons we have pop easily and don't provide much thrust below their bursting point. This is probably easier to fix (just buy new balloons).