The kV stays the same. You multiply the volts times the kV to get the unloaded RPMs.
48V X 67kV = 3216-RPM
60V X 67kV = 4020-RPM
The smaller RC motors will get very hot at high amps, and the heat will rise sharply and suddenly when you apply a load to it. They are made for a light model airplane that will accelerate to top-speed in a short time, then the plane will cruise at top-speed for a long time to allow the heat of acceleration to cool off. An E-bike is much heavier, and when you try to accelerate it will take longer, so the controller will apply many amps for a long time.
You can only apply 3200 watts to the small RC motor for a very short time.
You can apply 450W to the GNG all day long and it will only be warm. Most builders are applying 48V X 30A =1440W for fairly long amounts of time, because it can shed heat fairly well while you are riding.
The labels on both of these motors are poorly chosen. The GNG is a 1,000W system, and can use a temporary peak of 1,500W to accelerate as long as you do not apply 1,500W for too long. The 63mm RC motor is a 48V X 15A = 720W motor continuous, and capable of 1,000W temporary peak if you are using it on an E-bike (but...you must watch the temperatures very carefully, they get hot too fast with no warning). For an E-bike, the only RC motor I recommend is the 80mm diameter motors (with an air-fan attached to the shaft), they have more copper mass.
