Please consider the possibility that the car can be repaired in proportion to the percentage of parts currently available.
For example, if I need 40 parts for a full repair but only have 20, then I should be able to repair 50% of the car with those 20 parts. If I am “short” on parts, each race requires more and more, so in the end I have to buy tokens to repair the car.
I believe that proportional repair based on available parts would be fair.