Many questions

First of all welcome to the forum.

Second boost bottles dont add anything to these china girl engines so do one if you wish but the best engine builders and racers of these ebngine will all tell you a boost bottle is a waist of time and money.

As far as the 1/4 throttle you spoke of these untilize a short pull on the thottle but I guess you could make your own. If you wanted.

Go through this forum and use the search feature, you will find piles of info on performance tuning these engines and what works and what is snake oil like the boost bottles, if someone like the look of having the bottle on their bike that is fine but dont expect any performance gain from it.

Cleaned up ports, a well tuned carb and a good free flowing exhaust will do way more than gadgets like boost bottles on these engines, these china girls will never perform even close to a good Japanese 2 Stroke engine no matter whT you do to it, but there are several things that can be done to improve performance.

Best wishes and again wel ome to our great forum.

Map

Speed carb is big enough, unless you do major work.
any bigger and your just likely to get poor low down and hard to tune.
Like Map said, clean up the ports and get a better exhaust.
Next step is to get compression and squish right.

Yes Theon I agree, a KDX 200 or KTM 250 carb will be way to big for a china girl engine, some use 21 or 22mm Mikuni Carb on really well built engine and maybe even a 24mm on a really radical build but a 45+ mph set up can be done with an NT carb, I know because I have an engine that isn't super built, just some port work, a Puch hi hi 70cc head, manic port matched shorty intake and a well tuned NT or NT Speed carb. On the best day I saw 48mph on this bike and I dont have an expansion chamber exhaust on it either, just a home built straight through exhaust that is short and loud.....so far I haven't found anything that works better on my bikes than it does, every expansion pipe I've had on it so far decreased my top speed by several mph, im working on another that will flow better and hopefully help with better low and midrange without sacrificing the top end.....

We I said all of this to just give a little info on what some of us do to get a good strong running engine, some of it also has to do with whether or not the engine has correct timing which can vary up to about 6 degrees from what I've seen and whether or not the crank is very untrue or the balance of the rotating assembly is good enough to allow the engine to spin up to some fairly high rpms, I've seen 9400rpm on my fastest engine according to the tach I had on it I was running 40's mph with a 36T rear sprocket this bike gps' d 45.7 on the flat and I weighed 220 at the time, I have now slimmed down to 210lbs.....lol

Anyway my best advice is to do some research here on the forum to see what the people who have been doing this awhile are doing and have done and do things one step at a time....