You asked for detailed:
You simply can't have it both ways, good top speed AND reasonable take-off, [useing a stock engine] and a 30 dollar centrifigul clutch. You probably have a 60 tooth rear sprocket so with a 12 tooth on the clutch you have a 5 to 1 reduction. The engine turns over 5 times for evey revolution of the rear tire. However, as your engine is a 4-stroke it only provides 2-1/2 power impulses per revolution of the rear tire. If you are running only on a hard surface [concrete/blacktop] with very little stopping and starting and you do not weigh over 200 or so pounds the clutch will survive for a satisfactory period of time. If you are riding in the back yard on grass and have to stop and start a lot you will need at least a 72 tooth sprocket [6 to 1 ratio]. A 12 tooth and 60 tooth set-up will only work with approx. 6 inch rim, standard diameter tire and light load on harder ground. It is commonly the tallest ratio ever provided by a manufacturer. That should tell us something.
If it is important to go for high speed runs only then a taller ratio can be used but I have the feeling that 35 or so miles per hour is all you can expect with a govenor on a stock engine cutting in at about 3600 or 4000 RPM. Sure, you can make a mini go pretty fast even with a stock 5 horse, but you will largely end up with a "single purpose" machine.
The centrifigul clutch is in fact a clutch but it is really being used as a "torque converter". That is, it converts low torque and high RPM [from the engine] to high torque and low RPM at the rear tire. This is useful for taking -off. Unfortunately it does this rather inefficiently and produces copious amounts of heat in the process which will kill the clutch if it is slipped a great deal due to the gear ratio being grossly wrong for the type of riding being done.
Get a TAV and you can have both reasonable take-off and high speed. That will cost you about the price of your engine by the time you get through with eveything includeing a guard. The other way, without breaking the bank, is to install a jack shaft. With a sprocket change on the jack shaft you can, with a minimum of wrenching time, have a stump puller or a mini geared for the salt flat. Ther is such a thing as a two speed jack shaft. They tend to break and you are not gonna get all the parts needed for under 150 bucks.
There is, as in all things mechanical, No Free Lunch.