No I haven't given those any thought at all. with the CG 66cc gaining 2 hp over stock (for 4 or 4.5 hp total) is fairly easy and not terribly expensive. Real one horsepower gains after that come reluctantly and not in the form of pure bolt ons but as a result of tuning all the changes to work together. The best way to find hp/torque is by using real motorcycle race expansion pipes designed for small engines and these aren't bolt on and even after fabricating adapters to mount one on the China girl that just gets you to the point of tuning the pipe and engine setup to take advantage of the pipes dynamics. For example I used a KTM pipe on a ported and mildly tuned 66cc that would run 40mph easily.. and other than being really loud with the KTM pipe it was a disappointment, several weeks later I had it running great and the pipe was the key. Even then I doubt I gained more than 2 hp, but that was a huge bounce to performance. In the process I changed port timing, jetting & compression ratio (lowered it) The pipe ran better with a slightly shorter, 1 or 1.5" shorter, and 1/8" smaller diameter stinger. My elevation is at 1,500 ft.. I like to run just to the rich side with jetting just enough rich that pulling the air filter off brings idle up slightly.
I wish all these bolt ons would add 1/2 hp each 'cause I've always wanted a 66 cc China girl with 15 hp!
Rick C.