Good information can be had from "Oz" at Cycle Imports, tel. # (305) 597-4440. If your bike is a factory 75cc version, i.e. without the kit added to a 50cc bike the rear sprocket will have 42 teeth as opposed to 52.This could cause low top speed if running on the 52T. sprocket.
Apparently the factory produced 75cc. bike has a diferent cylinder, I believe the stroke has been changed so if you ever want to fit a cylinder with better porting, a 75cc. kit for a GPR 50 will not fit! Have not yet found a source for a performance kit for the GPR 75.
Another way to confirm a 75cc bike is obviously to measure the bore. This can be done without stripping the engine quite easily. Take two pieces of stiff wire bend one piece at 90" so that it measures 20mm(close enough for half the bore of 39.88mm for the 50, the other piece bend so that it measures 23.5mm. half of the 47mm. bore of the 75). Leave enough length on the long side so that you can't drop it into the cylinder! Remove the plug and insert the wire into the cylinder you should not be able to feel the cylinder wall with the shorter wire if it is a 75cc. machine. The longer wire will hit the wall with the vertical portion in the center of the plug hole, just make sure you measure near the top so you miss the ports.
My cylinder has the Derbi logo cast into it and a part # AO6982 stamped on the rear, and is a factory GPR 75.This bike was not purchased in the States, but I believe they are just being imported now. Oz would be a good person to call if you have any further questions or need parts.
Hope this is of some help.