Advice for racers and speed freaks:

Riding for top speed, whether in an official motorized bicycle race or just to impress your buddies, requires power to get you all the way up to the top speed. But before I go any farther I want you to pull out your mental garbage pan and toss everything you know from the motorcycling world concerning power for racing there. Why? Because motorcycles have gearboxes and these don't. Well, the exception to that is if you have a shift kit that incorporates a jackshaft so you can use your bicycle gears. But none of the official racers use it. Why? Because a bicycle shifter is clumsy and inaccurate compared to a motorcycle shifter, and the extra bearings and chain eats up 8% of the engine power. If you have one I feel sorry for you because you can do far more, far simpler with engine mods without shifting. I know because I proved that to myself when I lived in a hilly city at an altitude of 8500 ft. After the mods and a torque pipe I could climb any hill without having to have a big rear wheel sprocket (just a 36 tooth). Top speed was excellent also. All without shifting. Just a 48cc upgraded to 55cc, not even 66cc.
Normally a racing motorcycle needs an engine that focuses on top rpm power and they get quickly up to their top speed by keeping the engine in its peak rpm powerband while shifting through the gears. But if you have only one gear you need a different strategy. You need a torquey engine that will pull hard from idle to top rpm. With one gear and an engine that is tuned only for peak rpm power it will take you too long to finally get up into its powerband. With my 55cc it runs as good as a torquey engine with standard muffler exhaust pipe but then when it gets half way through its rpm range the torque pipe kicks in and the engine gets a surge of power that lasts all the way to peak rpm. That is exactly what is needed on these motorized bicycle race tracks since the bikes are normally at half speed at the apex of the turns and then they gas it coming out of them to progress up to top speed (45-50mph). Most motorized bicycle racers use the "shifting motorcycle" idealology because their bikes only have good top speed power which is a mistake.

Here's ideas for a "modern racer" w/o replacing the cylinder with a motorcycle one:
porting- reworked for racing, rear boost port added, transfers directed more rearward [link]
piston- motorcycle piston for more compression and power because of less ring end gap [link]
reed valve- Reed Valve Kit (available clamp on sizes 19/21/23mm) but if max RPM is less than 9000 you should replace the carbon fiber reeds with G10 Fiberglass Sheet .015 Inch from Radical RC
carburetor-  18mm Mikuni
head- stock one lathed down to provide up to 140psi
ignition- Jaguar CDI ignition system
pipe- my custom performance Pipe
engine oil- Maxima Castor 927 engine oil mixed 35:1
chain- heavy duty 415 chain
tires- Maxxis hookworms
extras- tachometer (RPM) gauge, head temp gauge