Re: Doing my first build. HOLY COW!
Unless you're an actual mechanic with very strong mechanical knowledge, get a good kit from a known supplier. I order mine from eBay, but I have a strong understanding of engines and all things mechanical. I tear them down all the way and clean out the dirt and shavings first. Then I meticulously inspect everything, clean up the ports, and check/adjust the crank trueness and balance. I'll say that 48cc engines are well balanced from the factory. 66cc are not so much.
Do not start buying bolt-on performance parts. Do that after getting the kit together. There is not much to upgrade in the stock carb. You get no extra noticeable power from any carb, and if you do it's because the stock one was jetted wrong. The only reason I went to a PZ16 carb was for the o-ring seal and tunable idle jet.
There are some expansion pipes that can produce power, but the cheap banana pipes aren't it. A re-worked pit bike expansion pipe will do the trick, but you're going to need a welder. I had good luck with making my own "poopoo pipe", but I must have hit the sweet spot without knowing it. I only made it so the exhaust would be behind me and not directed at my foot.
CDI boxes... You really cant get much better than stock. Unless you build and tune your own. Even then, you had better be knowledgeable with tuning and electronics to do so.
A good cylinder head can raise compression and give you some good power, but it's recommended to wait until 3 or 4 tanks of gas to ensure you have a good broken-in engine. Even then, you'll want a colder spark plug and possibly a higher grade octane fuel.
Do not start at 16:1 mix. It will oil soak your spark plug and fill your exhaust. I would start with 24:1, and after break in move to 36:1. That's always up for debate though.
Don't try to hit top speed right away. Don't take it easy, but not super hard either. You just need to vary the speed a lot while seating the rings.
Also, swap the spark plug for an NGK B6HS. You can get one at parts stores for under $3. Torque the head bolts to 120 in-lb, or 80 in-lb for 6mm studs (found on 48cc engines).
Never stop wrenching