Honestly, that "might" stop you from an outright win but don't panic.
1. Can you scramble and get a replacement somewhere fast?
a. Junkyard.
b. Hubcap Store.
c. Dealer.
2. Ok, so even if there is a possibility that you might not win...
so what! Go out there and strut your stuff! Show off that award winning shine, that gleaming paint, that black trim and that glistening chrome. Let the competition shudder as they smell the new car smell when you open your doors to show off your detailed dash or pristine carpet. Heck....
Just have fun!
That's what car shows are all about!
Your hub cap will just turn into a "hard luck story" and you know what, it may not even count against you.
Once I had an accident on the way to a concours where I ran over a dead animal. It caused severe damage to my Jag.

I couldn't fix it so I put a small sign by the dent which said "Oops". Believe it or not, no judge took points off for it. They all gave me mercy! I still won my class!
This could happen to you.