It's been asked over and over.
There are many vendors/sellers who sell these bicycle motor kits, which for the most part are all the same and all come from the same factory(s). Here's a list of several.... http://motorbicycling.com/f3/find-dealer-here-1730.html
There IS a difference though. If you just slap one on a bike and ride, you may get lucky and it will give you good service for a long time....but let's get real. These engine kits need to be set up with a little prep work http://motorbicycling.com/f30/bicycl...-care-354.html
and require at least minimal maintenance to give good service life.
Some sellers charge more for shipping than it actually costs 29.99 vs. 54.99 ect. Some will tell you their kits are 80cc and some will tell everyone there is no such thing. Some sellers will say they have a special or better set up than another seller....some do, but then again, some don't and say they do.
The bottom line here is this; Do a little homework, ask a few questions (not "which kit is best?") and know that when you buy a frame mount made in China kit, you are buying a pig in a poke so to speak. Most all are serviceable and will make you happy, a few won't.
DON'T buy one if you really really plan on commuting to work everyday and want to be sure you'll get there. Do buy a better, (rack mount) Honda or Subaru powered kit. Tanaka 2 stroke engines are the top of the line for 2 stroke rack mounts. The Dax Titan is a good four stroke rack mount and now also offers a friction drive and in frame version as well.
Whizzers are made in Taiwan, not the U.S. as in the old days and have problems of their own, so keep that in mind before you buy one because it's like the one Unka Pete had when you were a kid and always wanted one.
Buyer beware on any kit you buy from China.
The frame mount china engine kits are all a novelty if you will and will never be the same as a Fuji Robin (Subaru) or Honda engine kit.
If you like to tinker and/or experiment, then the Chinese two stroke frame mount kit is for you.
If you want a super reliable daily rider to go 20 miles to work on daily, spend a bit more and go for one that is made in Japan.