Just a comment, as fun as PWD can be, don't be that parent that clearly did all the work. The purpose is not to make a car NASA would be proud of, but to make the car with your son to end up with a car he made and is proud of.
Usually this results in a car that looks like and reflects the age of the scout (e.g. a Tiger Scout's (Kindergarten) car does not look like a Webelos (5th Grade) car.
Seriously suggest to your pack leadership that they add three races:
-Adult/Sibling -Adult Scouter -Adult Vintage
The first give you and your other non-scout kids an opportunity to be a part of things. You can make your NASA approved car that ultimately loses to the simple block of wood that five year old suzie makes.
The second gives the scouts a chance to cheer on their den leaders.
The third let's them see what PWD cars from pack in the day looked like.
Just some suggestions from an old Cubmaster.