Whose airplane is it anyway?

I have just been informed by the designer of the aircraft that I am
building that there is a possibility that he may not allow me to use an
alternative engine. Granted that weight and balance, as well as power
requirements must reasonably lie within allowable ranges, what is the
roll of the kit or plans manufacturer in the final homebuilt aircraft?
Will the FAA award him the power to veto my airworthiness certificate?