before anyone says multiplayer would be unfair, just do some server side checks to make sure the power to weight and such are similar to the allowed specs. Maybe it could be something like:
cars=XFG+XRG+FBM
power-to-weight-max=500
wing-allowed=yes/no
front-wing-allowed=yes/no
And of course, car editor would be licensed only... Maybe an S3 specific feature?