Selling assembled non-EPA approved 2 stroke bicycle engine kits is prohibited in CA, due to CARB regulations, but you can find kits from some vendors, and owning and riding them isn't illegal at all.
Four strokes are better for everyday use, and two strokes are better for a ripping good time and have more of a feel of a racing engine, that's why I like the two strokes for fun. Two strokes require more tuning and maintenance to keep them running at peak, but if you have a mellow tuned 66cc that's jetted right and doesn't lean out or run rich and foul plugs, it can be a reliable long distance runner as well. The main reliability problems from two stroke bicycle engines is from manufacturing defects, or assembly mistakes at the factory. There are vendors who address these issues and sell reliable bicycle engines, but it's up to you to tune them properly and keep an eye on them so they don't run with an air leak which can cause self destruction.
Two strokes are great for learning about tuning and performance, the small four strokes are good for touring without maintenance, and the big four stroke utility/minibike engines make a lot of power and can be extensively modded as well. Whichever you choose, know and assert your rights to ride, and don't bend under pressure, carry paperwork if its a local problem.
