All of the following information is based on my feeling that you have chosen to install an in-frame 2 stroke engine kit. If this is incorrect please let me know what kit you have decided on. To help unclutter your bars, here's one alternative. Check this link for a dual pull brake lever.> Sick Bike Parts
This combines front and rear brakes in one handlebar lever. They work great and are adjustable so you can choose the amount of force to each brake.
Now lets think about your gears. Many, not all, but a lot of bikes will require the removal of the front derailer which will eliminate shifting the front sprocket size. The reason is because the engine will be mounted right where the derailer mounts. You asked to be educated so let me inform you here that normal pedaling as you've come to expect it will not be possible due to gear drag through the engine. Even when the clutch is disengaged the clutch and gears will rotate with the rear wheel. There is an appreciable amout of drag induced. Yes, you will be able to ride as before installing the engine but be prepared for a little more effort. The alternative is to remove the engine drive chain if you want to pedal normally. You can do this by using a master link in the drive chain. Generally speaking most motorized bicycle riders do not rely on the pedals much after installing the engine. You pedal to get up to engine start speed, engage the clutch and the engine takes over. Pedaling is reduced to starting from a dead stop and 'helping' the engine on very steep hills if you gear for normal riding. There is a lot of good information available here so take your time, do some reading and all of your questions will be answered. If you need help on a specific area please feel free to ask in the appropriate forum. Hope this gets you started. As for sources for parts, check the sponsors you see down both sides of this page. They are all reliable dealers who have been proven to be reputable.