I'll give you a few reasons why you should NOT get a flybyebay engine kit.
As the forums have matured, many suppliers have gotten on board. Saving $10-50 through bidding may seem like a good thing at the beginning, then getting free advise on the forum to make up for kit deficiencies may cost you money in the long run.
But the sponsors are paying for space that allows the free advise to happen.
You are going to save money on gas irregardless of which you choose, if you buy from one of our sponsors, it will take you an extra couple of weeks.
By using somebody that is already established on the forum, they most likely have already fixed kit deficiencies, like better bolts, better mufflers, more secure shipping containers. Then when you install a kit, and detect something wrong, ESPECIALLY a manufacturing error, you help the sponsor by notifying him, so he can correct it.
A sponsor will fix things quicker, because they want to keep a good reputation. They won't make you a whiz bang mechanic, but chances are you'll get a lot better ride by following their advise.
A sponsor would also know more about his kit on your model of bike, while "flybyseatofpants" could care less what frame you use.
Finally, everybody on a motorized bike will tell you that your social life, talking to absolute strangers, is going to increase at every stop. "Where'd you buy it?" the 2nd or 3rd question. "I dunno" would be the e-bay answer, while using one of the sponsors, you know the first name of the owner.
Buy local is my motto.