Hi ........... One tooth less will give you more top speed (if you can pull top revs) but will reduce accelleration. If you go for one tooth less you'll need to tighten the chain and/or remove a link. It's a trade off ............ decide what you want the bike to do.
Not much to start with here unfortunatly mate. A free flow muffler will help top end speed, change the factory airbox to a K&N style pod air filter, other than that, hugging the tank at top speed :) If you do change these parts, you may also need to increase the main jet size to richen up the mixture for better tuning of the motor with the better flowing parts. This bike has a top speed of 100kmp and produces 10 hp, so you may only expect to get at best less than 1 extra neddy, and perhaps another 5 km/hr.... I suggest upgrading to a bigger machine of you feel the need for speed :)
If its 125's you are into, test drive a Honda CBR 125R 6 speed fuel injected 13.4 horsepower, and good for 120km/hr
Take the wheel off and remove the brake hub. Now clean out all the dirt and mud from riding in the creek and through mud puddles. The brake shaft has dried mud on it and can not rotate freely in the hub. You will probably need a plastic hammer to punch it out of the casing. clean the shaft and the casing hole the shaft goes through. Grease the shaft with waterproof grease.
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If all other electricals works then just replace the horn (hurn?)