I have bought kits from Spookytooth, zone8cycles, gasbikes.net, usabikemotors and thatsdax.
Thatsdax had the best price, but you have to be home for delivery and the chain tensioner is the older 2 bolt version, or mine was. Still on 1st gallon of gas.
Spookytooth was the most expensive and my first kit, they have since lowered their price. They have a web forum I hear getting them on the phone is sort of hard. Engine has over 1000 miles now, I think, my neighbor has it and rides it to work almost every day.
Zone8cycles reasonable price and shipping. Somewhere around 500-600 on engine miles.
Gasbike.net, I just don't trust them and got the runaround about what they say they were selling and what I got (they are also kingsmotorbike so I wonder why they need 2 sites selling identical products). I had to fix the centrifugal clutch
Usabikemotors also reasonable, but I have to pay sales tax since I live in AZ. Built for an acquaintance. Haven't seen him, he is pretty rough on it and it is still going.
In all cases there was some issue with each purchase. All issues were rectified without me having to pay more. In gasbikes case I just gave in to their ad being incorrect because it would have cost me too much to have it restocked.
Pay less than $200 with shipping and hope for the best. Expect something to be dented, broken or scratched