I'm sorry, but I just have to cringe when someone comes on without any exposure to motorized bicycles and with the intention of building and selling them. Build one for yourself first. Then another one. Then one more. Maybe a couple more after that. Then see how you feel about it.
Another obstacle is that for less than the price of a good quality complete motorized bicycle that goes almost 30 MPH, you can get a brand new 50cc scooter that does the exact same thing, except safer, more reliably, and often more legally (considering that the laws surrounding motorized bicycles are convoluted where they exist at all).
But, if you really want to start it up, you're going to want to import a pallet of engine kits from China, so you can pay wholesale price on them. That's about the only way to make this kind of venture profitable, and even then, you're not going to recoup that profit until you turn all those kits. Sure, there are people who manage shops, but they're in temperate places where you can ride year round, not north of Chicago.