just my own humble opinion, kits are not the only way to go with a bicycle. there are a lot of ways to motorize a bike without building from a kit, and some of those options give much better performance than a kit supplied engine. the kit engines simply are not designed for a wide range of performance. there are some options available to increase the performance range of a kit supplied engine, some are very expensive, and some are not. but for the same amount of money you could likely spend hopping up a kit, you could build a bike with a better performing engine. you can buy a kit without a motor and find a way to adapt most of the parts to a non-kit engine. one engine that is slowly but steadily becoming more popular is the predator engine, available at harbor freight stores. I am not recommending that you choose to install a predator engine, or even any non-kit engine. a lot of people are happy with the performance of the kit supplied engines or are willing to spend the money to buy performance enhancing parts. I am simply suggesting that you explore all your options and ask around and let others know what you are looking for, such as what top speed you want, what sort of hill climbing or pulling ability you need, what sort of reliability you would be comfortable with, and so forth. you will likely get some different recommendations, but I can tell you right off the bat that a 4 stroke engine will be more reliable than a 2 stroke, but the 2 stroke engines and kits are less expensive. there are a lot of options out there, and I hope you find the option that best suits your wants and needs.