Ok, here is how that clutch works. It is called a centrifugal clutch, and it is called that because it works by centrifugal force. Centrifugal force is when any object inside a spinning circle is pushed toward the outside of the circle. The clutch has shoes and a spring. The spring is designed to expand with a certain amount of pressure, translated to rpm's of the engine (the faster the engine spins, the more pressure is put on the spring). The spring keeps the shoes from spinning out against the drum until it reaches the required amount of pressure to allow it to expand. This is how an engine is able to idle with a centrifugal clutch. There are different springs that allow the clutch to engage at different rpm's. A lighter spring allows for earlier engagement, while a heavier spring allows for later engagement. There are also different weights of shoes that affect clutch engagement, but in the opposite way. A lighter shoe allows for later engagement, but a heavier shoe allows for earlier engagement. From what I have read, a heavy spring and a heavy shoe is the best way to have smooth engagement at almost any rpm. Others have found success with light shoes and a light spring. I'm currently running light shoes and a heavy spring, but getting ready to order some heavier shoes as soon as I have the money. On the subject of jackshafts, a jackshaft is essentially a shaft that transfers driving force from one side of a shaft to the other. It can be a 1:1 jackshaft (same size gear on both sides of shaft) as in the case of the shift kits, or it can be different size gears to achieve a desired amount of reduction, which is how they are normally used on 4 strokes. By reduction, I mean the relationship between the engine speed and the rear wheel speed. 4 strokes need added reduction because they do not have any built-in reduction like the kit 2 stroke engines do, and they are too high revving for direct drive (direct drive is when you have the same size drive and driven gears, and no other gears in between. The maximum suitable rpm range for direct drive is 1500-1800. The harbor freight engines operate at a maximum of about 6000 rpm's). With a jackshaft, you can pretty much choose what size gears you want for your desired ratio. The gear ratio determines the top speed and hill climbing ability of your bike. My bike, which has a 98cc Lifan minibike engine, winds out at about 5000 rpm's and with a gear ratio of 12.44:1, tops out around 32 mph. The 79cc engine winds out at about 6000 rpm's with the governor removed, and with the same gear ratio as my bike will go about 40 mph. You can gear it lower for less top speed and more hill climbing ability. If I ever happen to use a 79cc predator, I plan to gear it down to about 30 mph as well. So you see, there is no real "basic" answer, because you have to decide what you want and find out what of the available options will get you what you want. There are too many different ways to build a 4 stroke, and each one will produce different results as far as looks, speed, handling, and general riding style. Its best to get a general idea of what you want, then compare other's builds to see which ones come closest to what you want, not to copy anyone's build, but just to get a general idea of how to go about building the bike that you want. Because you may realize as I did, once you get into a custom build, its about way more than just how the bike gets from a to b. Although that is the most important part, you are building a custom bike so you might as well customize the whole bike to your own personal needs and wants. You've got a 2 stroke to putt-putt around on, so take your time and plan all the details. Then you'll know what parts you'll want and need, and you'll be able to take your time, progressing as you get the parts you want. Rather than starting out with something basic and only changing things as you go along, which can be a lot more expensive than just building it right the first time. When I built my custom bike, I started with a bare frame and gathered up parts one at a time, not actually building the bike until I had all the parts on the list. I've changed and added a few things since then, but I started with the bike and the parts I wanted.
